[core] Refactor OpenGL extension loading mechanism

Previously, we initialized global variables that held pointers to the extension functions. While this seemed to work, the spec doesn't guarantee that the function pointers are identical for different OpenGL contexts. Therefore, we are now making them a member variable of the Context object.

src/mbgl/gl/vertex_array.cpp

@@ -1,22 +0,0 @@
-#include <mbgl/gl/vertex_array.hpp>
-
-namespace mbgl {
-namespace gl {
-
-ExtensionFunction<void(GLuint array)>
-    BindVertexArray({ { "GL_ARB_vertex_array_object", "glBindVertexArray" },
-                      { "GL_OES_vertex_array_object", "glBindVertexArrayOES" },
-                      { "GL_APPLE_vertex_array_object", "glBindVertexArrayAPPLE" } });
-
-ExtensionFunction<void(GLsizei n, const GLuint* arrays)>
-    DeleteVertexArrays({ { "GL_ARB_vertex_array_object", "glDeleteVertexArrays" },
-                         { "GL_OES_vertex_array_object", "glDeleteVertexArraysOES" },
-                         { "GL_APPLE_vertex_array_object", "glDeleteVertexArraysAPPLE" } });
-
-ExtensionFunction<void(GLsizei n, GLuint* arrays)>
-    GenVertexArrays({ { "GL_ARB_vertex_array_object", "glGenVertexArrays" },
-                      { "GL_OES_vertex_array_object", "glGenVertexArraysOES" },
-                      { "GL_APPLE_vertex_array_object", "glGenVertexArraysAPPLE" } });
-
-} // namespace gl
-} // namespace mbgl