iris: Handle vertex shader with window space position

Iris advertises support for PIPE_CAP_TGSI_VS_WINDOW_SPACE_POSITION
so let's actually implement it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110657

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
13 jobs for !12 with fix/iris/handle-window-space-position in 12 minutes and 9 seconds (queued for 11 minutes and 54 seconds)
merge request