[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, 46 insertions, 0 deletions

diff --git a/src/mbgl/programs/attributes.hpp b/src/mbgl/programs/attributes.hpp
index c4cc5dea8b..bb90f2c13c 100644
--- a/src/mbgl/programs/attributes.hpp
+++ b/src/mbgl/programs/attributes.hpp
@@ -64,6 +64,34 @@ struct a_color : gl::Attribute<gl::Normalized<uint8_t>, 4> {
     }
 };
 
+// used in the symbol sdf shader
+struct a_fill_color : gl::Attribute<gl::Normalized<uint8_t>, 4> {
+    static auto name() { return "a_fill_color"; }
+    
+    static Value value(const Color& color) {
+        return {{
+            gl::Normalized<uint8_t>(color.r),
+            gl::Normalized<uint8_t>(color.g),
+            gl::Normalized<uint8_t>(color.b),
+            gl::Normalized<uint8_t>(color.a)
+        }};
+    }
+};
+
+// used in the symbol sdf shader
+struct a_halo_color : gl::Attribute<gl::Normalized<uint8_t>, 4> {
+    static auto name() { return "a_halo_color"; }
+    
+    static Value value(const Color& color) {
+        return {{
+            gl::Normalized<uint8_t>(color.r),
+            gl::Normalized<uint8_t>(color.g),
+            gl::Normalized<uint8_t>(color.b),
+            gl::Normalized<uint8_t>(color.a)
+        }};
+    }
+};
+
 struct a_stroke_color : gl::Attribute<gl::Normalized<uint8_t>, 4> {
     static auto name() { return "a_stroke_color"; }
 
@@ -171,5 +199,23 @@ struct a_offset<1> : gl::Attribute<float, 1> {
     }
 };
 
+struct a_halo_width : gl::Attribute<float, 1> {
+    static auto name() { return "a_halo_width"; }
+    
+    static Value value(float width) {
+        return {{ width }};
+    }
+};
+
+struct a_halo_blur : gl::Attribute<float, 1> {
+    static auto name() { return "a_halo_blur"; }
+    
+    static Value value(float blur) {
+        return {{ blur }};
+    }
+};
+
+
+
 } // namespace attributes
 } // namespace mbgl