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>

src/compiler/shader_info.h

@@ -149,6 +149,9 @@ typedef struct shader_info {
       struct {
          /* Which inputs are doubles */
          uint64_t double_inputs;
+
+         /* True if the shader writes position in window space coordinates pre-transform */
+         bool window_space_position;
       } vs;
 
       struct {