shader: track variable sinfo with the shader variant

Gert Wollny requested to merge gerddie/virglrenderer:fix-neverball into master

Up until now sme of the info stored in sinfo was changed based on the shader key, and since this info is used for all shader variants when evaluating the key for shaders to be linked, this may result in the wrong shader variant being picked up.

Instead track the shader info that can be changed like this in the shader variant itself.

Closes: #239 (closed)

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