WIP: shader: Use a temp output when alpha-test code is injected

Gert Wollny requested to merge gerddie/virglrenderer:work-around-lockup into master

Otherwise we end up with reading from an output, and this triggers enabling sample shading on MS surfaces in IRIS, and this may lead to a GPU hang.

I put this on WIP, because I think the better fix should be in mesa (i.e. not seting the uses_sample_shading property early.

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