Generally, most servers send hextile updates as single updates
containing many rects. Some servers send hextile updates as many small
framebuffer updates with a few rects each (such as QEMU). This latter
cases revealed that shifting off the beginning of the receive queue
(which happens after each hextile FBU) performs poorly.

This change switches to using an indexed receive queue (instead of
actually shifting off the array). When the receive queue has grown to
a certain size, then it is compacted all at once.

The code is not as clean, but this change results in more than 2X
speedup under Chrome for the pessimal case and 10-20% in firefox.