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.

Servers/WindowServer/WSCompositor.h

@@ -40,6 +40,7 @@ private:
     unsigned m_compose_count { 0 };
     unsigned m_flush_count { 0 };
     CTimer m_compose_timer;
+    CTimer m_immediate_compose_timer;
     bool m_flash_flush { false };
     bool m_buffers_are_flipped { false };