WSCompositor: Allow a compose to bypass the timer when it first happens

d66fa60fcf introduced the use of a timer
to coalesce screen updates. This is OK, but it does introduce update
latency.

To help mitigate the impact of this, we now have a second (immediate)
timer. When a compose pass is first triggered, the immediate timer will
allow the compose to happen on the next spin of the event loop (so, only
coalescing updates across a single event loop pass). Any updates that
trigger while the delayed timer is running, though, will be delayed to
that (~60fps) timer.

This fixes #103.

SharedGraphics/DisjointRectSet.h

@@ -12,6 +12,7 @@ public:
     void add(const Rect&);
 
     bool is_empty() const { return m_rects.is_empty(); }
+    int size() const { return m_rects.size(); }
 
     void clear() { m_rects.clear(); }
     void clear_with_capacity() { m_rects.clear_with_capacity(); }