Drawable Renderer (#1570), work on fill drawables

include/mbgl/gfx/drawable_builder.hpp

@@ -4,6 +4,7 @@
 #include <mbgl/gfx/types.hpp>
 #include <mbgl/gfx/vertex_attribute.hpp>
 
+#include <array>
 #include <memory>
 #include <vector>
 
@@ -95,6 +96,13 @@ public:
     /// Add a rectangle consisting of two triangles
     void addQuad(int16_t x0, int16_t y0, int16_t x1, int16_t y1);
 
+    /// Add already-set-up triangles directly
+    
+    void addTriangles(const std::vector<std::array<int16_t,2>>& vertices,
+                      std::size_t vertexOffset, std::size_t vertexLength,
+                      const std::vector<uint16_t>& indexes,
+                      std::size_t indexOffset, std::size_t indexLength);
+
     /// Add a tweaker to be attached to each emitted drawable
     void addTweaker(DrawableTweakerPtr tweaker) { tweakers.emplace_back(std::move(tweaker)); }