[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.

test/api/custom_layer.test.cpp

@@ -1,9 +1,10 @@
 #include <mbgl/test/util.hpp>
 
-#include <mbgl/gl/gl.hpp>
+#include <mbgl/platform/gl_functions.hpp>
 #include <mbgl/map/map.hpp>
 #include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/storage/default_file_source.hpp>
+#include <mbgl/gl/defines.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
 #include <mbgl/style/style.hpp>
 #include <mbgl/style/layers/custom_layer.hpp>
@@ -14,6 +15,7 @@
 
 using namespace mbgl;
 using namespace mbgl::style;
+using namespace mbgl::platform;
 
 // Note that custom layers need to draw geometry with a z value of 1 to take advantage of
 // depth-based fragment culling.