mesa/st: add tgsi-lowering code for depth-clamp

mesa/st: add tgsi-lowering code for depth-clamp

This is a TGSI pass that lowers depth-clamping into shader-operations,
by replacing the depth-value with 0 (a z-coordinate of zero will always
pass the OpenGL depth test conditions), and using a dedicated varying to
interpolate the real depth-value instead. Finally we replace the
depth-output in the fragment shader.

v1 implemented by Erik Faye-Lund <erik.faye-lund@collabora.com>
v2: Add support for handling depth clip mode, and refactor code
v3: - Rename *_vs functions to *_last_vertex_stage (Erik)
    - Use 0.0 depth to avoid clipping (Erik)
v4: Fix inversion of bool value for clip control property

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Reviewed-by: Marek Olšák <marek.olsak@amd.com>