Fix so many things that there is little place in this column to list them all but the result is that blending modes just work now for drawing primitives.

Fixes involved:
1. Fix handling of alpha channel when SDL_BLENDMODE_NONE is set.
2. Make xrendercolor use floating-point values for color channels and then convert to 16 bit ints.
3. Fix handling of visuals in SDL_x11modes.c so that a 32 bit ARGB visual is used.
4. Fix the background pixel value in SDL_x11window.c so that the window background has an alpha value of 0xFF and not 0.