[core] Update icon shader code

Ported the following patch:
- [convert mat4 exMatrix to a vec2 extrudeScale](https://github.com/mapbox/mapbox-gl-shaders/commit/a8d549b7a41540d3a99767975ff1b7b18a6010e9)

4 files changed, 21 insertions, 34 deletions

diff --git a/package.json b/package.json
index 8eb1b16f7a..8f2a6ec852 100644
--- a/package.json
+++ b/package.json
@@ -19,7 +19,7 @@
   "devDependencies": {
     "aws-sdk": "^2.3.5",
     "express": "^4.11.1",
-    "mapbox-gl-shaders": "mapbox/mapbox-gl-shaders#13aad76282c1b6c4d38fd46f373325dfec69ab01",
+    "mapbox-gl-shaders": "mapbox/mapbox-gl-shaders#49022d5b92c067b8659f5700a04f376d75f2950e",
     "mapbox-gl-test-suite": "mapbox/mapbox-gl-test-suite#b3441d9a285ffbe9b876677acb13d7df07e5b975",
     "node-gyp": "^3.3.1",
     "request": "^2.72.0",
diff --git a/src/mbgl/renderer/painter.cpp b/src/mbgl/renderer/painter.cpp
index 0f7d894e02..d8636a9f11 100644
--- a/src/mbgl/renderer/painter.cpp
+++ b/src/mbgl/renderer/painter.cpp
@@ -100,7 +100,9 @@ void Painter::render(const Style& style, const FrameData& frame_, SpriteAtlas& a
     // The extrusion matrix.
     matrix::ortho(extrudeMatrix, 0, state.getWidth(), state.getHeight(), 0, 0, -1);
 
-    extrudeScale = {{ (2.0f / state.getWidth()) * state.getAltitude(), -2.0f / state.getHeight() * state.getAltitude() }};
+    const float flippedY = state.getViewportMode() == ViewportMode::FlippedY;
+    extrudeScale = {{ 2.0f / state.getWidth() * state.getAltitude(),
+                      (flippedY ? 2.0f : -2.0f) / state.getHeight() * state.getAltitude() }};
 
     // The native matrix is a 1:1 matrix that paints the coordinates at the
     // same screen position as the vertex specifies.
diff --git a/src/mbgl/renderer/painter_symbol.cpp b/src/mbgl/renderer/painter_symbol.cpp
index 59a543a987..6f628b9d16 100644
--- a/src/mbgl/renderer/painter_symbol.cpp
+++ b/src/mbgl/renderer/painter_symbol.cpp
@@ -183,35 +183,21 @@ void Painter::renderSymbol(SymbolBucket& bucket,
             mat4 vtxMatrix =
                 translatedMatrix(matrix, paint.iconTranslate, tileID, paint.iconTranslateAnchor);
 
-            bool skewed = layout.iconRotationAlignment == RotationAlignmentType::Map;
-            mat4 exMatrix;
-            float s;
-
-            if (skewed) {
-                matrix::identity(exMatrix);
-                s = tileID.pixelsToTileUnits(1, state.getZoom());
+            float scale = fontScale;
+            std::array<float, 2> exScale = extrudeScale;
+            const bool alignedWithMap = layout.iconRotationAlignment == RotationAlignmentType::Map;
+            if (alignedWithMap) {
+                scale *= tileID.pixelsToTileUnits(1, state.getZoom());
+                exScale.fill(scale);
             } else {
-                exMatrix = extrudeMatrix;
-                matrix::rotate_z(exMatrix, exMatrix, state.getNorthOrientationAngle());
-                s = state.getAltitude();
+                exScale = {{ exScale[0] * scale, exScale[1] * scale }};
             }
-            const bool flippedY = !skewed && state.getViewportMode() == ViewportMode::FlippedY;
-            matrix::scale(exMatrix, exMatrix, s, flippedY ? -s : s, 1);
-
-            matrix::scale(exMatrix, exMatrix, fontScale, fontScale, 1.0f);
-
-            // calculate how much longer the real world distance is at the top of the screen
-            // than at the middle of the screen.
-            float topedgelength = std::sqrt(std::pow(state.getHeight(), 2) / 4.0f * (1.0f + std::pow(state.getAltitude(), 2)));
-            float x = state.getHeight() / 2.0f * std::tan(state.getPitch());
-            float extra = (topedgelength + x) / topedgelength - 1;
 
             config.program = iconShader->getID();
             iconShader->u_matrix = vtxMatrix;
-            iconShader->u_exmatrix = exMatrix;
+            iconShader->u_extrude_scale = exScale;
             iconShader->u_texsize = {{ float(activeSpriteAtlas->getWidth()) / 4.0f, float(activeSpriteAtlas->getHeight()) / 4.0f }};
-            iconShader->u_skewed = skewed;
-            iconShader->u_extra = extra;
+            iconShader->u_skewed = alignedWithMap;
             iconShader->u_texture = 0;
 
             // adjust min/max zooms for variable font sies
diff --git a/src/mbgl/shader/icon_shader.hpp b/src/mbgl/shader/icon_shader.hpp
index 6e52f48aab..3388720da9 100644
--- a/src/mbgl/shader/icon_shader.hpp
+++ b/src/mbgl/shader/icon_shader.hpp
@@ -11,15 +11,14 @@ public:
 
     void bind(GLbyte *offset) final;
 
-    UniformMatrix<4>                u_matrix      = {"u_matrix",      *this};
-    UniformMatrix<4>                u_exmatrix    = {"u_exmatrix",    *this};
-    Uniform<GLfloat>                u_zoom        = {"u_zoom",        *this};
-    Uniform<GLfloat>                u_opacity     = {"u_opacity",     *this};
-    Uniform<std::array<GLfloat, 2>> u_texsize     = {"u_texsize",     *this};
-    Uniform<GLint>                  u_skewed      = {"u_skewed",      *this};
-    Uniform<GLfloat>                u_extra       = {"u_extra",       *this};
-    Uniform<GLint>                  u_texture     = {"u_texture",     *this};
-    Uniform<GLint>                  u_fadetexture = {"u_fadetexture", *this};
+    UniformMatrix<4>                u_matrix        = {"u_matrix",        *this};
+    Uniform<std::array<GLfloat, 2>> u_extrude_scale = {"u_extrude_scale", *this};
+    Uniform<GLfloat>                u_zoom          = {"u_zoom",          *this};
+    Uniform<GLfloat>                u_opacity       = {"u_opacity",       *this};
+    Uniform<std::array<GLfloat, 2>> u_texsize       = {"u_texsize",       *this};
+    Uniform<GLint>                  u_skewed        = {"u_skewed",        *this};
+    Uniform<GLint>                  u_texture       = {"u_texture",       *this};
+    Uniform<GLint>                  u_fadetexture   = {"u_fadetexture",   *this};
 
 protected:
     GLint a_offset = -1;