MythRender: Add a Release method to MythRenderOpenGL.

MythRenderOpenGL re-implements the QGLContext makeCurrent and
doneCurrent methods in order to enforce thread safety. We recursively
lock the context and only release it when the final doneCurrent call is
received. When we call show() on the UI painter window, however, Qt
calls makeCurrent several times internally without matching doneCurrent
calls and hence we never actually unlock and release the main OpenGL
context in normal use.

With the imminent move to a single, shared OpenGL context for video and
UI, this will cause a lockup at any point the decoder thread tries to
re-initiate the video output objects (or deinterlacers) when using
OpenGL for both video and the UI.

So we simply force the context to release itself after the call to
show().

mythtv/libs/libmythui/mythmainwindow.cpp

@@ -534,6 +534,8 @@ void MythMainWindow::ShowPainterWindow(void)
 {
     if (d->paintwin)
         d->paintwin->show();
+    if (d->render)
+        d->render->Release();
 }
 
 void MythMainWindow::HidePainterWindow(void)

mythtv/libs/libmythui/mythrender_base.h

@@ -51,6 +51,7 @@ class MythRender
     RenderType Type(void)       { return m_type;    }
     bool  IsErrored(void) const { return m_errored; }
     QSize GetSize(void) const   { return m_size;    }
+    virtual void Release(void)  { }
 
   protected:
     virtual  ~MythRender() { }

mythtv/libs/libmythui/mythrender_opengl.cpp

@@ -100,6 +100,12 @@ void MythRenderOpenGL::doneCurrent()
     m_lock->unlock();
 }
 
+void MythRenderOpenGL::Release(void)
+{
+    while (m_lock_level > 0)
+        doneCurrent();
+}
+
 void MythRenderOpenGL::MoveResizeWindow(const QRect &rect)
 {
     QWidget *parent = (QWidget*)this->device();

mythtv/libs/libmythui/mythrender_opengl.h

@@ -107,6 +107,7 @@ class MUI_PUBLIC MythRenderOpenGL : public QGLContext, public MythRender
 
     virtual void makeCurrent();
     virtual void doneCurrent();
+    virtual void Release(void);
 
     void  Init(void);