[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/gl/context.test.cpp

@@ -1,10 +1,11 @@
 #include <mbgl/test/util.hpp>
 
-#include <mbgl/gl/gl.hpp>
+#include <mbgl/platform/gl_functions.hpp>
 #include <mbgl/gl/context.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>
@@ -16,6 +17,7 @@
 
 using namespace mbgl;
 using namespace mbgl::style;
+using namespace mbgl::platform;
 
 static const GLchar* vertexShaderSource = R"MBGL_SHADER(
 #ifdef GL_ES