Revert two shader layout changes to fix regressed dEQP tests

Ryan Neph requested to merge ryanneph/virglrenderer:revert-shader-layout into master

Reverts two commits to fix the regressed dEQP tests described #266 (closed):

c5f118c8ff1 2022-06-02 shader: Remove layout_location field         [needed to fix the build after previous revert]
99f8cd3dc0c 2022-05-26 shader: remove obsolete layout handling code [bisection flagged as the cause of regression]

If we can't arrive at a solution to #266 (closed), I'd request landing this revert to fix the tests in the meantime.

/cc @gerddie @zzyiwei @chadversary

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