X.org on Amiga, 16 color mode

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Hi all,

I was looking into how to get the x.org server running with color
on non-AGA Amigas. As you may remember, it is currently working
with 256 colors on AGA-equipped Amigas and black/white on others.

Looks like the server wants to use 8bpp memory layout for depth 4
(16 colors), defined in xf86Init.c formats array. This differs from
old Xamiga which used 4bpp with depth 4.

Working solution seems to be creating the shadowfb with 8bpp and
depth 4, and then handling conversion from 8bpp pixels to 4
bitplanes in the shadow update function.

For the conversion it needs a new shadow update function, which is
pretty similar to existing shadowUpdateAfb4, just reading 8-bit
input pixels instead of 4-bit pixels.

Patch attached. Any thoughts?

--jukka

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