Make GdkGLTexture determine if the texture has
a mipmap, and provide private API to query this
information.
This check is done in gdkgltexture.c instead of
gskgldriver.c, since we're already binding the
texture here for other reasons, so it is easy
to query a few more things.
This is necessary so that bidnings work properly and don't make
gdk_gl_texture_release() a function on GdkTexture.
It also allows code to identify what type of texture they are dealing
with.
Finally, we can now decide to add getters later without screwing
anything up, if we want to allow people to access GL textures directly.
Put GdkGLTexture into its own file and rename the API to
gdk_gl_texture_foo() instead of gdk_texture_foo_for_gl().
Apart from naming, no actual code changes.
