Since we're really only initializing grabs (except for implicit
grabs at least) from the client side we might as well do all the grab
time checks on the client side to avoid unnecassary roundtrips.
Ensure that we're writing all the updates for a single rendering op
in one go without callbacks inbetween. That way some rendering will
be delayed, but the user will never see partial renderings.
The true diff only works if the destination keeps perfect 32bit
canvas data, which is not always true. So, instead we send only
changed pixels, masking the others to 0 via alpha 0.
We put this in gtk+ for now since it will be synced initially, but
it avoids using glib so that it can be used lower in the stack if
required (e.g. in cairo later).
