`set_alpha` does turn on `_force_alpha` -- in fact, it already did, I didn't need to change anything there. (But prior these changes, practically nothing actually paid attention to it.) Calling `gc.set_alpha(None)` clears `_forced_alpha`.

This doesn't break the non-forced alpha case because the backends themselves, where appropriate, now pull the alpha value straight out of `_rgb[3]` in the non forced alpha case.