Fix regression: when a palette + colorkey is converted, it needs a blend mode.

- Regression of test_1.c of bug 3827, after fix from bug 4798.
- Blending is also needed when the palette contains alpha value, but not necessarily colorkey.
- Clean up SDL_ConvertColorkeyToAlpha which doesn't seem to need 'ignore_alpha' parameter any-more.

(see bug 3827)
1 file changed