vcl: only use default windows' GL Context for bitmaps if NULL / uninitialized.

Change-Id: I6465c548ba5d50da2cca7cce24e9bd2a20b28f47

vcl/opengl/salbmp.cxx

@@ -464,14 +464,17 @@ sal_uInt16 OpenGLSalBitmap::GetBitCount() const
 
 bool OpenGLSalBitmap::makeCurrent()
 {
-    OpenGLContextProvider *pProvider;
-    pProvider = dynamic_cast< OpenGLContextProvider* >( ImplGetDefaultWindow()->GetGraphics() );
-    if( pProvider == NULL )
+    if (!mpContext || !mpContext->isInitialized())
     {
-        SAL_WARN( "vcl.opengl", "Couldn't get default OpenGL context provider" );
-        return false;
+        OpenGLContextProvider *pProvider;
+        pProvider = dynamic_cast< OpenGLContextProvider* >( ImplGetDefaultWindow()->GetGraphics() );
+        if( pProvider == NULL )
+        {
+            SAL_WARN( "vcl.opengl", "Couldn't get default OpenGL context provider" );
+            return false;
+        }
+        mpContext = pProvider->GetOpenGLContext();
     }
-    mpContext = pProvider->GetOpenGLContext();
     mpContext->makeCurrent();
     return true;
 }