Add screen recorder tool

Add screen recorder tool

This commit adds a new, built-in screen recorder tool.  The tool UI is
still very simple, start with mod-r and stop it again with mod-r.
The recording is written to capture.wcap, in a simple run-length encoded
adhoc format.  The wcap-decode tool can be used to extract a single frame
from the capture, for now, but the plan is to hook this up to libvpx and
generate webm output.