glsl: add NV_viewport_array2 support

This enables gl_Layer/gl_ViewportIndex when the ext is enabled, as well
as adding the new gl_ViewportMask[] array and viewport_relative layout
qualifier for gl_Layer.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4529>

src/compiler/shader_info.h

@@ -191,6 +191,9 @@ typedef struct shader_info {
    /* Whether the shader writes memory, including transform feedback. */
    bool writes_memory:1;
 
+   /* Whether gl_Layer is viewport-relative */
+   bool layer_viewport_relative:1;
+
    union {
       struct {
          /* Which inputs are doubles */