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>
9 files changed