mesa: add PackedDriverUniformStorage const

Will be used to determine whether to take packing code paths or not.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/mtypes.h

@@ -4132,6 +4132,9 @@ struct gl_constants
 
    /** GL_ARB_get_program_binary */
    GLuint NumProgramBinaryFormats;
+
+   /** Is the drivers uniform storage packed or padded to 16 bytes. */
+   bool PackedDriverUniformStorage;
 };