shader,feat: Add support for TGSI_PROPERTY_FS_DEPTH_LAYOUT

Gert Wollny requested to merge gerddie/virglrenderer:fix-property-6 into master

GL 4.2 supports depth layouts that make it possible to optimize by enabling early depth tests and still being able to write a new z value in the fragment shader under specific circumstances.

Fixes #106 (closed)

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