To get truly atomic updates, add a mechanism for passing arbitrary amounts
of extra data along with WindowServer messages. This allows us to pass all
the rects in a single message.
This patch moves to sending up to 32 rects at a time when coordinating the
painting between WindowServer and its clients. Rects are also merged into
a minimal DisjointRectSet on the server side before painting.
Interactive resize looks a lot better after this change, since we can
usually do all the repainting needed in one go.
