Created attachment 13817 [details]
Program reproducing the problem with RGBA mode.

The packed RGBA Xv mode (id 0x41424752) gives weird results when used
on a PPC machine with a Radeon 9200 graphics card (Mac Mini).  The green
component is decoded as expected, but the red and blue are read twice from
each word, giving half the X resolution.

I've attached a program reproducing the problem.  It divides the window
into a 3x3 grid, painting the squares along the diagonal in solid red,
green, and blue, respectively.  Which is to say, the expected result is this:

R__  (R = red, G = green, B = blue, _ = black)
_G_
__B

Instead, what you get is this

RR_
_G_
____BB

(The blue square ends up outside the window, so it isn't seen.)