[core] Add DDS support for {text,icon}-size (#8593)

* Update gl-js and generate style code

* Factor out packUint8Pair() helper function

* Draft implementation of DDS for {text,icon}-size

Ports https://github.com/mapbox/mapbox-gl-js/pull/4455

* Fix text-size/composite-function-line-placement test

* Refactor to PaintPropertyBinders-like strategy

* Dedupe gl::Program construction

* Use exponential function base for interpolation

* Dedupe coveringZoomStops method

* Fixup tests

* Fix CI errors (hidden within #if block)

1 files changed, 44 insertions, 7 deletions

diff --git a/src/mbgl/shaders/symbol_icon.cpp b/src/mbgl/shaders/symbol_icon.cpp
index 9adda0ba16..b6fbed428e 100644
--- a/src/mbgl/shaders/symbol_icon.cpp
+++ b/src/mbgl/shaders/symbol_icon.cpp
@@ -9,9 +9,16 @@ const char* symbol_icon::name = "symbol_icon";
 const char* symbol_icon::vertexSource = R"MBGL_SHADER(
 
 attribute vec4 a_pos_offset;
-attribute vec2 a_texture_pos;
 attribute vec4 a_data;
 
+// icon-size data (see symbol_sdf.vertex.glsl for more)
+attribute vec3 a_size;
+uniform bool u_is_size_zoom_constant;
+uniform bool u_is_size_feature_constant;
+uniform mediump float u_size_t; // used to interpolate between zoom stops when size is a composite function
+uniform mediump float u_size; // used when size is both zoom and feature constant
+uniform mediump float u_layout_size; // used when size is feature constant
+
 uniform lowp float a_opacity_t;
 attribute lowp vec2 a_opacity;
 varying lowp float opacity;
@@ -19,6 +26,7 @@ varying lowp float opacity;
 // matrix is for the vertex position.
 uniform mat4 u_matrix;
 
+uniform bool u_is_text;
 uniform mediump float u_zoom;
 uniform bool u_rotate_with_map;
 uniform vec2 u_extrude_scale;
@@ -34,16 +42,45 @@ void main() {
     vec2 a_pos = a_pos_offset.xy;
     vec2 a_offset = a_pos_offset.zw;
 
-    vec2 a_tex = a_texture_pos.xy;
-    mediump float a_labelminzoom = a_data[0];
-    mediump vec2 a_zoom = a_data.pq;
+    vec2 a_tex = a_data.xy;
+    mediump vec2 label_data = unpack_float(a_data[2]);
+    mediump float a_labelminzoom = label_data[0];
+    mediump vec2 a_zoom = unpack_float(a_data[3]);
     mediump float a_minzoom = a_zoom[0];
     mediump float a_maxzoom = a_zoom[1];
 
-    // u_zoom is the current zoom level adjusted for the change in font size
-    mediump float z = 2.0 - step(a_minzoom, u_zoom) - (1.0 - step(a_maxzoom, u_zoom));
+    float size;
+    // In order to accommodate placing labels around corners in
+    // symbol-placement: line, each glyph in a label could have multiple
+    // "quad"s only one of which should be shown at a given zoom level.
+    // The min/max zoom assigned to each quad is based on the font size at
+    // the vector tile's zoom level, which might be different than at the
+    // currently rendered zoom level if text-size is zoom-dependent.
+    // Thus, we compensate for this difference by calculating an adjustment
+    // based on the scale of rendered text size relative to layout text size.
+    mediump float layoutSize;
+    if (!u_is_size_zoom_constant && !u_is_size_feature_constant) {
+        size = mix(a_size[0], a_size[1], u_size_t) / 10.0;
+        layoutSize = a_size[2] / 10.0;
+    } else if (u_is_size_zoom_constant && !u_is_size_feature_constant) {
+        size = a_size[0] / 10.0;
+        layoutSize = size;
+    } else if (!u_is_size_zoom_constant && u_is_size_feature_constant) {
+        size = u_size;
+        layoutSize = u_layout_size;
+    } else {
+        size = u_size;
+        layoutSize = u_size;
+    }
+
+    float fontScale = u_is_text ? size / 24.0 : size;
+
+    mediump float zoomAdjust = log2(size / layoutSize);
+    mediump float adjustedZoom = (u_zoom - zoomAdjust) * 10.0;
+    // result: z = 0 if a_minzoom <= adjustedZoom < a_maxzoom, and 1 otherwise
+    mediump float z = 2.0 - step(a_minzoom, adjustedZoom) - (1.0 - step(a_maxzoom, adjustedZoom));
 
-    vec2 extrude = u_extrude_scale * (a_offset / 64.0);
+    vec2 extrude = fontScale * u_extrude_scale * (a_offset / 64.0);
     if (u_rotate_with_map) {
         gl_Position = u_matrix * vec4(a_pos + extrude, 0, 1);
         gl_Position.z += z * gl_Position.w;