mesa: rework ctx->Driver.CopyTexSubImage() parameters

mesa: rework ctx->Driver.CopyTexSubImage() parameters

Replace target, level parameters with gl_texture_image.
Add gl_renderbuffer parameter to indicate source buffer for the copy.

This removes some redundant code in the drivers to find the source
renderbuffer and the destination texture image (which we already had
in _mesa_CopyTexSubImage).

Signed-off-by: Brian Paul <brianp@vmware.com>