Change ImplDrawAlpha to DrawAlphaBitmap

OutputDevice has a public function ImplDrawAlpha, but that just draws
a bitmap with an alpha layer. It's really a public function, though
not yet used outside of OutputDevice. However, the name should really
be the same as SalGraphic's function which is DrawAlphaBitmap.

Change-Id: If2716c1a1a194079d6514a1d30d8b05f9fda664c