glsl,nir: Switch the enum representing shader image formats to PIPE_FORMAT.

This means you can directly use format utils on it without having to have
your own GL enum to number-of-components switch statement (or whatever) in
your vulkan backend.

Thanks to imirkin for fixing up the nouveau driver (and a couple of core
details).

This fixes the computed qualifiers for EXT_shader_image_load_store's
non-integer sizeNxM qualifiers, which we don't have tests for.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com> (v3d)
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3355>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3355>

src/compiler/glsl/ir.h

@@ -29,6 +29,7 @@
 #include <stdlib.h>
 
 #include "util/ralloc.h"
+#include "util/format/u_format.h"
 #include "compiler/glsl_types.h"
 #include "list.h"
 #include "ir_visitor.h"
@@ -885,8 +886,11 @@ public:
       uint8_t warn_extension_index;
 
    public:
-      /** Image internal format if specified explicitly, otherwise GL_NONE. */
-      uint16_t image_format;
+      /**
+       * Image internal format if specified explicitly, otherwise
+       * PIPE_FORMAT_NONE.
+       */
+      enum pipe_format image_format;
 
    private:
       /**