XWindowsEventQueueBuffer: Fix delays when waiting for new events

QLength() may return 0 even if there are events pending because they
need to be read from the display socket in order to become visible. We
must use XPending() which will poll the socket if QLength() == 0.

src/lib/platform/XWindowsEventQueueBuffer.cpp

@@ -82,6 +82,12 @@ XWindowsEventQueueBuffer::~XWindowsEventQueueBuffer()
     close(m_pipefd[1]);
 }
 
+int XWindowsEventQueueBuffer::getPendingCountLocked()
+{
+    Lock lock(&m_mutex);
+    return XPending(m_display);
+}
+
 void
 XWindowsEventQueueBuffer::waitForEvent(double dtimeout)
 {
@@ -163,7 +169,7 @@ XWindowsEventQueueBuffer::waitForEvent(double dtimeout)
     // we want to give the cpu a chance s owe up this to 25ms
 #define TIMEOUT_DELAY 25
 
-    while (((dtimeout < 0.0) || (remaining > 0)) && QLength(m_display)==0 && retval==0){
+    while (((dtimeout < 0.0) || (remaining > 0)) && getPendingCountLocked() == 0 && retval == 0) {
 #if HAVE_POLL
     retval = poll(pfds, 2, TIMEOUT_DELAY); //16ms = 60hz, but we make it > to play nicely with the cpu
      if (pfds[1].revents & POLLIN) {

src/lib/platform/XWindowsEventQueueBuffer.h

@@ -51,6 +51,8 @@ class XWindowsEventQueueBuffer : public IEventQueueBuffer {
 private:
     void                flush();
 
+    int getPendingCountLocked();
+
 private:
     typedef std::vector<XEvent> EventList;
      IXWindowsImpl*       m_impl;