fix X11 video backend

fix X11 video backend

when launching xgnuboy, i got the following error:

X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  1 (X_CreateWindow)
  Serial number of failed request:  7
  Current serial number in output stream:  11

comparing the X startup code with suckless' st, the only major
difference i could find was that st uses XDefaultDepth() which returns
24, whereas gnuboy tried 32 bit before 24 bit and for some reason the
call to XMatchVisualInfo succeeded with 32, however apparently it then
failed to create the window, while it worked perfectly fine when
re-arranging the struct to have 24 bit first.

i modified the loop now to try whatever the defaultdepth is first, and
if that fails loop over all depths as done previously.