Filter out alpha channel for normal draw() operation

Filter out alpha channel for normal draw() operation

macOS actually uses the alpha channel on windows, so we can get visual
artifacts if we feed it bogus alpha data. This filtering unfortunately
causes some CPU usage, but it's necessary until we can make sure the
framebuffer always contains proper 0xff for alpha.

(cherry picked from commit a88991bdc8154bda8a805755dd82cb71c93b35df)