GUACAMOLE_377: Read lock should NOT be acquired at same time as write…

… lock.

Acquiring the read lock first and then reentrantly acquiring the write
lock may result in other existing readers getting promoted to writers.

src/libguac/display-flush.c

@@ -272,7 +272,7 @@ static int PFW_LFW_guac_display_frame_complete(guac_display* display) {
 
 void guac_display_end_mouse_frame(guac_display* display) {
 
-    guac_rwlock_acquire_read_lock(&display->pending_frame.lock);
+    guac_rwlock_acquire_write_lock(&display->pending_frame.lock);
 
     if (!display->pending_frame_dirty_excluding_mouse)
         guac_display_end_multiple_frames(display, 0);