v3d: reinterpret stencil data as uint texture in stencil blit path

There is a path to blit stencil buffers reinterpreting the stencil data as an RGBA8888 or R8 float texture.

This works fine except for the case when the stencil buffer is multisampled, and the blit operation needs to resolve it: an average of the samples is done, which is incorrect, as only one sample must be used.

This can be observed n the piglit test ext_framebuffer_multisample-unaligned-blit 2 stencil downsample -auto -fbo, specifically in the triangles border.

To avoid this averaging, let's reinterpret the stencil data as RGBA8888 or R8 uint texture.

This fix come from !8305 (closed)