Protect PlatformPixelBuffer from simultaneous access

The damage tracking region needs to be protected from multiple
threads accessing it at once. The rest should be fine though.
diff --git a/vncviewer/PlatformPixelBuffer.cxx b/vncviewer/PlatformPixelBuffer.cxx
index 5bd50d2..522bad3 100644
--- a/vncviewer/PlatformPixelBuffer.cxx
+++ b/vncviewer/PlatformPixelBuffer.cxx
@@ -28,15 +28,19 @@
 void PlatformPixelBuffer::commitBufferRW(const rfb::Rect& r)
 {
   FullFramePixelBuffer::commitBufferRW(r);
+  mutex.lock();
   damage.assign_union(rfb::Region(r));
+  mutex.unlock();
 }
 
 rfb::Rect PlatformPixelBuffer::getDamage(void)
 {
   rfb::Rect r;
 
+  mutex.lock();
   r = damage.get_bounding_rect();
   damage.clear();
+  mutex.unlock();
 
   return r;
 }