textscreen: Init screen at native bit depth.

Always call SDL_SetVideoMode with bpp=0 to use the desktop's native
bit depth, and instead draw into an intermediate 8-bit screenbuffer
surface. This ensures that we do not encounter any palette mess-ups
as on some systems true 8-bit screen modes are poorly supported.
hexen: Create HR (startup) screen at native bpp.

Always call SDL_SetVideoMode() with bpp=0 to use the native pixel
depth of the display. This avoids problems with some systems that
don't properly support 8-bit screen modes any more. Draw into an
intermediate buffer and let SDL take care of the pixel depth
conversion for us.