st/mesa: implement EGLImageTargetTexStorage

We can now support this extension.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3375>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3375>
2 files changed