glsl/linker: add xfb workaround for modified built-in variables

glsl/linker: add xfb workaround for modified built-in variables

Some lowering passes modify the value of built-in variables in
order for drivers to work properly. However, modifying such values
will also break transform feedback as the captured value won't
match what's expected.

For example, on some hardware, the vertex shaders are expected to
output gl_Position in screen space. However, the transform
feedback captured value is still supposed to be the world-space
coordinates (see nir_lower_viewport_transform).

To fix that, we create a new variable that contains the
pre-transformation value and use it for transform feedback instead
of the built-in one.

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2433>