surface-actor: Prevent a dumb crash

is_argb32 can be called at any time, including times when we don't
have a texture. In that case, just assume we're ARGB32. The value
really shouldn't be important though.
surface-actor: Implement is_argb32 generically for both X11 and Wayland

cogl_texture_get_components can be used on both X11 and Wayland
backends. Technically, the detection is different: we actually
check the actual RENDER format in the old code, while Cogl simply
assumes that any pixmap with a depth >= 32 is ARGB32. Since Cogl
already seems to be working with its internal checks, it makes
more sense to  use Cogl's check rather than keeping our own.