[GRASS5] XDRIVER_TRUECOLOR

[Huidae Cho]

I found an undocumented environment variable "XDRIVER_TRUECOLOR" in
display/drivers/XDRIVER/XDRIVER24/Graph_Set.c.  Shouldn't this be renamed
to GRASS_TRUECOLOR?

I'm just curious about:

1. why XDRIVER doesn't support GRASS_BACKGROUNDCOLOR
2. why d.erase wipes out the monitor with DEFAULT_BG_COLOR by default,
   not by GRASS_BACKGROUNDCOLOR
3. why we don't have GRASS_FOREGROUNDCOLOR

Thanks.
Huidae Cho