[macOS] support OpenGL < 4.2 in PBS shader (#377)

- OpenGL < 4.2 does not support swizzling built in types (e.g. float).
- Update the definition of @piece( maxR1F0 ) to work in OpenGL 4.1

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

ogre2/src/media/Hlms/Pbs/GLSL/BRDFs_piece_ps.glsl

@@ -12,7 +12,7 @@
 @property( !fresnel_scalar )
 	@piece( maxR1F0 )max( 1.0 - ROUGHNESS, @insertpiece( F0 ).x )@end
 @end @property( fresnel_scalar )
-	@piece( maxR1F0 )max( (1.0 - ROUGHNESS).xxx, @insertpiece( F0 ).xyz )@end
+	@piece( maxR1F0 )max( (1.0 - vec3(ROUGHNESS, 0.0, 0.0)).xxx, @insertpiece( F0 ).xyz )@end
 @end
 
 //For mortals: