st/nine: Do not allow depth buffer render targets

Without the proposed check, some apps will decide to use depth buffers
as render targets.

Bug found investigating:
https://github.com/iXit/wine-nine-standalone/issues/82

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Acked-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9177>
1 file changed