Bug 1271 - mingw compiling failed: undefined reference to

	Bug 1271 - mingw compiling failed: undefined reference to
	`_glDrawRangeElements@24'

	Resolve glDrawRangeElements with cogl_get_proc_address instead of
	calling it directly because functions defined in GL > 1.1 are not
	directly exported under Windows.

	* clutter/cogl/common/cogl-mesh.c: Use the function pointer from
	the context

	* clutter/cogl/gl/cogl-context.c (cogl_create_context): Initialise
	function pointer.

	* clutter/cogl/gl/cogl-context.h (CoglContext): Add a function
	pointer

	* clutter/cogl/gl/cogl-defines.h.in: Add a typedef for the
	function pointer.

	* clutter/cogl/gl/cogl.c (_cogl_features_init): Resolve
	glDrawRangeElements