Hi!
I sent you current saturn.c. And i describe the problem in this email. Hope you will find the bug.
In two words, somehow with removage of my glitched off-screen buffer (i.e. bringing video routine to close to original emu) - the new glitch appeared - the background is in black. All background uses first color from palette. Somehow it's setten as black. But with usage of off-screen buffer it's not allow it to be setted as black.
Possible there also some problems with priority/transparency (not sure, they are in render.c and OS independend)