The JIT generated FS shader has logic to obey sample mask when:
multisample is enabled, or multisample is disabled but FS writes sample
mask and pipe_rasterizer_state::no_ms_sample_mask_out.
However it did not handle the case where multisample was disabled, FS
did not write sample_mask, and sample mask was zero. Instead it relied
upon the setup to discard the primitives, but that went away with commit
da5840f3.
We could restore the discard on zero mask behavior, but we would again
blurring the semantics of rasterization discard. Instead this change
adds logic to primitive setup to cull the primitives when sample mask is
zero.
Fixes: da5840f3 ("llvmpipe: Faithfully honour pipe_rasterizer_state::rasterizer_discard flag")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20934>
77092ca8f4