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.
diff --git a/src/compositor.h b/src/compositor.h
index 4a659bc..5752e81 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -124,6 +124,7 @@
 	struct weston_output_zoom zoom;
 	int dirty;
 	struct wl_list read_pixels_list;
+	struct wl_signal frame_signal;
 
 	char *make, *model;
 	uint32_t subpixel;