This patch adds a 512 frame timeline to Magnifier and the ability to
step through it with the arrow keys.
This makes it easier to check Serenity animations frame by frame for
correctness etc.
This adds an option for even more magnification, when you really need to
count pixels, as well as pausing the capture by pressing Space and
switching between magnification levels with keys 2, 4 & 8.
This patch moves the magnifier rect computation over to the server side
to ensure that the mouse cursor position and the screen image never get
out of sync.
