Gallium: add lowering of depth clamp for drivers that don't support it and enable it for virgl

Gert Wollny requested to merge gerddie/mesa:gallium-lower-depth-clamp into master

This series implements lowering of depth clamp in TGSI to provide ARB_depth_clamp/EXT_depth_clamp on hardware that doesn't support it. Most of the lowering code was implemented by @kusma and I've added support for GS and TES shaders and the interaction with ARB_clip_control.

Additional piglits to verify this code are provided by piglit!99

Edited by Gert Wollny

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