[core] Per-segment-per-layer vertex arrays

Reduces rebinding, matches gl-js, and works around the buggy VAO implementation on PowerVR SGX544 GPUs.

src/mbgl/programs/program.hpp

@@ -58,7 +58,8 @@ public:
               const SegmentVector<Attributes>& segments,
               const PaintPropertyBinders& paintPropertyBinders,
               const typename PaintProperties::Evaluated& currentProperties,
-              float currentZoom) {
+              float currentZoom,
+              const std::string& layerID) {
         typename AllUniforms::Values allUniformValues = uniformValues
             .concat(paintPropertyBinders.uniformValues(currentZoom, currentProperties));
 
@@ -66,8 +67,10 @@ public:
             .concat(paintPropertyBinders.attributeBindings(currentProperties));
 
         for (auto& segment : segments) {
-            if (!segment.vertexArray) {
-                segment.vertexArray = context.createVertexArray();
+            optional<gl::VertexArray>& vertexArray = segment.vertexArrays[layerID];
+
+            if (!vertexArray) {
+                vertexArray = context.createVertexArray();
             }
 
             program.draw(
@@ -77,7 +80,7 @@ public:
                 std::move(stencilMode),
                 std::move(colorMode),
                 allUniformValues,
-                *segment.vertexArray,
+                *vertexArray,
                 Attributes::offsetBindings(allAttributeBindings, segment.vertexOffset),
                 indexBuffer,
                 segment.indexOffset,

src/mbgl/programs/segment.hpp

@@ -28,7 +28,14 @@ public:
     std::size_t vertexLength;
     std::size_t indexLength;
 
-    mutable optional<gl::VertexArray> vertexArray;
+    // One VertexArray per layer ID. This minimizes rebinding in cases where
+    // several layers share buckets but have different sets of active attributes.
+    // This can happen:
+    //   * when two layers have the same layout properties, but differing
+    //     data-driven paint properties
+    //   * when two fill layers have the same layout properties, but one
+    //     uses fill-color and the other uses fill-pattern
+    mutable std::map<std::string, optional<gl::VertexArray>> vertexArrays;
 };
 
 template <class Attributes>

src/mbgl/programs/symbol_program.hpp

@@ -360,7 +360,8 @@ public:
               const SegmentVector<Attributes>& segments,
               const PaintPropertyBinders& paintPropertyBinders,
               const typename PaintProperties::Evaluated& currentProperties,
-              float currentZoom) {
+              float currentZoom,
+              const std::string& layerID) {
         typename AllUniforms::Values allUniformValues = uniformValues
             .concat(symbolSizeBinder.uniformValues(currentZoom))
             .concat(paintPropertyBinders.uniformValues(currentZoom, currentProperties));
@@ -370,8 +371,10 @@ public:
             .concat(paintPropertyBinders.attributeBindings(currentProperties));
 
         for (auto& segment : segments) {
-            if (!segment.vertexArray) {
-                segment.vertexArray = context.createVertexArray();
+            optional<gl::VertexArray>& vertexArray = segment.vertexArrays[layerID];
+
+            if (!vertexArray) {
+                vertexArray = context.createVertexArray();
             }
 
             program.draw(
@@ -381,7 +384,7 @@ public:
                 std::move(stencilMode),
                 std::move(colorMode),
                 allUniformValues,
-                *segment.vertexArray,
+                *vertexArray,
                 Attributes::offsetBindings(allAttributeBindings, segment.vertexOffset),
                 indexBuffer,
                 segment.indexOffset,