mesa: Don't call GenerateMipmap if Width or Height == 0.

mesa: Don't call GenerateMipmap if Width or Height == 0.

One of the WebGL 2.0 conformance tests is trying to call
glGenerateMipmaps with a width and height of 0.  With the meta
implementation, this generates a "framebuffer attachment incomplete"
status, and falls back to the CPU path, calling MapTextureImage.

Except that there's no actual texture to map, and we assert fail.

There's no work to do in this case.  The test expects it to succeed,
so just return early with no error and avoid hassling the driver.

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96911
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>