[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, 13 insertions, 4 deletions

diff --git a/src/mbgl/layout/symbol_feature.hpp b/src/mbgl/layout/symbol_feature.hpp
index e55995f952..f4dc1680bc 100644
--- a/src/mbgl/layout/symbol_feature.hpp
+++ b/src/mbgl/layout/symbol_feature.hpp
@@ -8,14 +8,23 @@
 
 namespace mbgl {
 
-class SymbolFeature {
+class SymbolFeature : public GeometryTileFeature {
 public:
-    FeatureType type;
+    SymbolFeature(std::unique_ptr<GeometryTileFeature> feature_) :
+        feature(std::move(feature_)),
+        geometry(feature->getGeometries()) // we need a mutable copy of the geometry for mergeLines()
+    {}
+    
+    FeatureType getType() const override { return feature->getType(); }
+    optional<Value> getValue(const std::string& key) const override { return feature->getValue(key); };
+    std::unordered_map<std::string,Value> getProperties() const override { return feature->getProperties(); };
+    optional<FeatureIdentifier> getID() const override { return feature->getID(); };
+    GeometryCollection getGeometries() const override { return geometry; };
+
+    std::unique_ptr<GeometryTileFeature> feature;
     GeometryCollection geometry;
     optional<std::u16string> text;
     optional<std::string> icon;
-    std::array<float, 2> iconOffset;
-    float iconRotation;
     std::size_t index;
 };