llvmpipe/ms: fix sign extension bug in rasterizer.

llvmpipe/ms: fix sign extension bug in rasterizer.

/glcts --deqp-surface-width=1024 --deqp-surface-height=64  --deqp-case=KHR-GL45.texture_view.view_sampling  --deqp-surface-type=fbo
was failing but only for width 1024.

The test was filling a 4x4 ms texture, but leaving the viewport set to 1024x64.

This was resulting in this code incorrectly sign extending a value, and passing
it into the mask generator and getting the wrong values. Explicit cast
avoids the sign extension and fixes the above test.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6006>