tdf#96385 - opengl: dynamically adjust priority of swap buffers.

Initially we start with a very low priority, so that the lame
bits of code that do eg. focus, and cursor rendering before the
document is visible do not cause a swap, flash. Then after we've
processed a REPAINT priority idle (hopefully our first paint) we
adjust the swap priority to highest.

Essentially a fusion of Tor's approach and mine.

Change-Id: Ib9b78a18fb9359ac8c3dee1cfeb30177c08fe162
Reviewed-on: https://gerrit.libreoffice.org/21405Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>

vcl/opengl/gdiimpl.cxx

@@ -47,7 +47,8 @@ public:
         : Idle( "gl idle swap" )
         , m_pImpl( pImpl )
     {
-        SetPriority( SchedulerPriority::HIGHEST );
+        // We don't want to be swapping before we've painted.
+        SetPriority( SchedulerPriority::POST_PAINT );
     }
     virtual ~OpenGLFlushIdle()
     {
@@ -55,6 +56,7 @@ public:
     virtual void Invoke() override
     {
         m_pImpl->doFlush();
+        SetPriority( SchedulerPriority::HIGHEST );
         Stop();
     }
 };