When performing a MSAA resolve in the blitter, either a linear
(nearest filter) or a bilinear (linear filter) average sampling is done
for float-based color sources.

In the case of V3D, there's a path to do a stencil blit by
re-interpreting the stencil buffer as a color buffer, and using the
blitter to do the job. But the BlitFramebuffer spec requires that for
stencil values a single sample is used, not an average.

This can be observed in the piglit's test
`ext_framebuffer_multisample-unaligned-blit 2 stencil downsample -auto
-fbo`, specifically in the triangles borders.

Here we add an option to disable the averaging and use instead a single
sample.

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>