reading overlay planes on non-PI machines

[Kurt Akeley]

readback of the overlay/underlay/popup bitplanes is supported for GT, GTX,
and VGX machines in the 3.3 release.  it is documented on the readsource
manpage, but it is complicated.  basically, you can ignore readsource,
as lrectread will return pixels from the currently active framebuffer,
as specified by the drawmode command.  therefore, to read or write pixels
in the overlay bitplanes, simply set drawmode to OVERDRAW and call the
appropriate read or write commands.  likewise for underlay and popup
bitplanes.  most pixel reading applications can now ignore readsource.

-- kurt

p.s. readback of overlay/underlay/popup bitplanes in this manner is also
     supported by the 3.3 personal iris release.  it also continues to
     support readsources SRC_OVER, SRC_UNDER, and SRC_PUP, which will not
     be implemented on any other machines.