[core] Switch to the new OpenGL abstraction

This patch will make Mapbox GL Core never use OpenGL
directly. We should consider locking into OpenGL ES 2.0
to simplify the code path and remove #ifdefs.

src/mbgl/gl/attribute.cpp

@@ -1,10 +1,12 @@
 #include <mbgl/gl/attribute.hpp>
 #include <mbgl/gl/context.hpp>
-#include <mbgl/gl/gl.hpp>
+#include <mbgl/gl/defines.hpp>
 
 namespace mbgl {
 namespace gl {
 
+using namespace platform;
+
 void bindAttributeLocation(Context& context, ProgramID id, AttributeLocation location, const char* name) {
     // We're using sequentially numberered attribute locations starting with 0. Therefore we can use
     // the location as a proxy for the number of attributes.