namcos21: clear screen to palette index 0 instead of 0xff

[FlykeSpice]

This makes the sky background from Winning Run games to blend more seamless with the fading horizon (specially winrungp and winrun91), this is just a duct-tape fix and the actual clear screen behavior from the hardware needs to be studied.

[angelosa]

I tested solvalou / winrungp / cybsled, there are some extra flags in the unused section of palette RAM (that is RGB888). I don't think it's alpha, more like bit selects for vblank / framebuffer erase stuff?

[angelosa]

Won't break anything else because winrun and friends uses their own screen_update compared to the other variants. Adding a note for unknown bits 2-1 in unused paletteram is a nice to have.