gl: Add a private way to query mipmap status

gl: Add a private way to query mipmap status

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.