[core] Implement data-driven styling for {text,icon}-{color,opacity,halo-color,halo-blur,halo-width} (#7939)

* Add symbol dds attributes and adapt style code generation

* Update to mapbox-gl-js/master

* Refactor SymbolFeature as a subclass of GeometryTileFeature

Prepares for enabling DDS on symbol paint properties by allowing the
SymbolFeatures, which we keep around after constructing SymbolLayout,
to be used in evaluating data-driven paint properties later in the
layout process.

* Draft approach for splitting icon/text paint properties

The `Program` types are set up to bind GL attributes to each of the
data-driven paint properties specified in the `PaintProperties` type
provided.  Since `SymbolPaintProperties` specifies both `Text*` and
`Icon*` properties, the symbolIcon, symbolIconSDF, and symbolGlyph
programs each attempt to bind roughly double the number of attributes
that they actually need.

This change addresses this by:
- Adding the more specific `IconPaintProperties` and `TextPaintProperties` types, which are subsets of the full `SymbolPaintProperties`.
- The symbol layer continues to use its `SymbolPaintProperties paint` member to track layer property state, but it provides helpers that construct objects of each the specific `{Icon,Text}PaintProperties::Evaluated` type, for use by the painter.
- The three symbol programs instantiate `Program<>` using the appropriate `{Icon,Text}PaintProperties` type.

* check in generated style code

* Populate paint buffers for symbol DDS properties

* Address first round of review comments

* Refactor VectorTile{Layer,Feature} to explicitly share data

* Update submodule

1 files changed, 11 insertions, 2 deletions

diff --git a/src/mbgl/shaders/symbol_icon.cpp b/src/mbgl/shaders/symbol_icon.cpp
index eca9342d54..e6728e15de 100644
--- a/src/mbgl/shaders/symbol_icon.cpp
+++ b/src/mbgl/shaders/symbol_icon.cpp
@@ -66,6 +66,10 @@ attribute vec2 a_offset;
 attribute vec2 a_texture_pos;
 attribute vec4 a_data;
 
+uniform lowp float a_opacity_t;
+attribute lowp float a_opacity_min;
+attribute lowp float a_opacity_max;
+varying lowp float opacity;
 
 // matrix is for the vertex position.
 uniform mat4 u_matrix;
@@ -80,6 +84,8 @@ varying vec2 v_tex;
 varying vec2 v_fade_tex;
 
 void main() {
+    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
+
     vec2 a_tex = a_texture_pos.xy;
     mediump float a_labelminzoom = a_data[0];
     mediump vec2 a_zoom = a_data.pq;
@@ -122,13 +128,16 @@ precision mediump float;
 #endif
 uniform sampler2D u_texture;
 uniform sampler2D u_fadetexture;
-uniform lowp float u_opacity;
+
+varying lowp float opacity;
 
 varying vec2 v_tex;
 varying vec2 v_fade_tex;
 
 void main() {
-    lowp float alpha = texture2D(u_fadetexture, v_fade_tex).a * u_opacity;
+    
+
+    lowp float alpha = texture2D(u_fadetexture, v_fade_tex).a * opacity;
     gl_FragColor = texture2D(u_texture, v_tex) * alpha;
 
 #ifdef OVERDRAW_INSPECTOR