swrast: Convert color glReadPixels slow path to using MapRenderbuffer.

swrast: Convert color glReadPixels slow path to using MapRenderbuffer.

This may be a bit slower than before because we're switching from
per-format compiled loops in GetRow to
_mesa_unpack_rgba_block_unpack's loop around a callback to unpack a
pixel.  The solution there would be to make _mesa_unpack_rgba_block
fold the span loop into the format handlers.

(On the other hand, function call overhead will hardly matter if
MapRenderbuffer means the driver gets the data into cacheable memory
instead of uncached).

The adjust_colors code should no longer be required, since the unpack
function does the 565 to float conversion in a single pass instead of
converting it (poorly) through 8888 as apparently happened in the
past.

Reviewed-by: Brian Paul <brianp@vmware.com>