nir: Add ability for shaders to use window space coordinates.

nir: Add ability for shaders to use window space coordinates.

This patch adds a shader_info field that tells the driver to use window
space coordinates for a given vertex shader. It also enables this feature
in radeonsi (the only NIR-capable driver that supported it in TGSI),
and makes tgsi_to_nir aware of it.

Signed-Off-By: Timur Kristóf <timur.kristof@gmail.com>
Tested-by: Andre Heider <a.heider@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Eric Anholt <eric@anholt.net>