llvmpipe: add array/3d clearing support

Fixes:
dEQP-VK.pipeline.multisample.sampled_image.64x64_4.*.samples_4

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6381>
1 file changed