Prefer it to VK_MVK_macos_surface when present.

MoltenVK has deprecated VK_MVK_macos_surface in favor of
VK_EXT_metal_surface. It's likely that the former extension will vanish
at some point.
Signed-off-by: Chip Davis <cdavis@codeweavers.com>
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>