glthread: use GLenum16 for enums, but clamp it to 0xffff to get correct errors

glthread: use GLenum16 for enums, but clamp it to 0xffff to get correct errors

0xffff is an invalid enum.

This was initially implemented without clamping and then reverted because
of the concern that we would randomly get correct enums and not report
errors if we just dropped the high bits.

This packs calls better in glthread_batch.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18828>