Backport r27281 from trunk to release-0-24-fixes

Don't call QGLContext::doneCurrent() in MythRenderOpenGL.

    * it should be unnecessary given that makeCurrent always checks the
correct context is current to the current thread and following the
threading changes, all of our OpenGL usage should be in the same thread.
    * it breaks playback on certain Windows builds.
    * it has a sizeable performance impact on at least one Intel system.

Closes #9242


git-svn-id: http://svn.mythtv.org/svn/branches/release-0-24-fixes@27292 7dbf422c-18fa-0310-86e9-fd20926502f2

mythtv/libs/libmythui/mythrender_opengl.cpp

@@ -138,9 +138,11 @@ void MythRenderOpenGL::makeCurrent()
 
 void MythRenderOpenGL::doneCurrent()
 {
+    // we don't explicitly call QGlContext::doneCurrent() as it should be
+    // unnecessary (makeCurrent() will switch contexts as necessary), it
+    // appears to cause performance issues and breaks rendering in certain
+    // situations
     m_lock_level--;
-    if (m_lock_level == 0)
-        QGLContext::doneCurrent();
     if (m_lock_level < 0)
         VERBOSE(VB_IMPORTANT, LOC_ERR + "Mis-matched calls to makeCurrent()");
     m_lock->unlock();