shader: rework and fix cull and clip distance handling

Merged Gert Wollny requested to merge gerddie/virglrenderer:fix-clip-and-cull into master

The series starts with a few code cleanups that helped identifying the issues with the clip and cull distances.

The actual fix is then to make the TGSI properties take precedence for CLIP and CULL and fixing the swizzling when reading the values in the FS.

Edited by Gert Wollny

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