mesa: (more) correctly handle incomplete depth textures

mesa: (more) correctly handle incomplete depth textures

according to GL spec, incomplete shadow samplers should return 0

this is technically possible for drivers to do using a RGBA texture in
the sense that somehow it's been working, but it's broken at the gallium-level
for what drivers should be expecting to see in such circumstances given
that such scenarios have been binding a RGBA texture to use with shadow samplers

instead, we can give drivers a fallback Z32 texture to avoid format/sampler
mismatches and complying with expected behavior

see also KHR-GL46.incomplete_texture_access.sampler for driver-specific testing

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20817>