ARB_gl_spirv xfb improvement

This series started as just trying to use the general NIR xfb gathering pass, added for Vulkan consumption, and ended fixing some corner cases on such gathering, and on the ARB_gl_spirv support for xfb itself. All that can be tested on piglit here: https://github.com/Igalia/piglit/tree/apinheiro/xfb

For now a personal branch. It is mostly done, but it is pending the double checking before sending to review.

As part of this series, we also change how input arrays of blocks are splitted, in order to support interpolators qualifiers for such case.