mesa: add UseSTD430AsDefaultPacking constant

This will be used to enable the STD430 layout as the default for
UBOs and SSBOs with layouts of shared/packed rather than STD140.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/mtypes.h

@@ -3843,6 +3843,15 @@ struct gl_constants
     */
    GLboolean DisableVaryingPacking;
 
+   /**
+    * UBOs and SSBOs can be packed tightly by the OpenGL implementation when
+    * layout is set as shared (the default) or packed. However most Mesa drivers
+    * just use STD140 for these layouts. This flag allows drivers to use STD430
+    * for packed and shared layouts which allows arrays to be packed more
+    * tightly.
+    */
+   bool UseSTD430AsDefaultPacking;
+
    /**
     * Should meaningful names be generated for compiler temporary variables?
     *