[macosx 10.15] Fix for https://github.com/openscenegraph/OpenSceneGraph/issues/926 from @PetrBo and @tikittel

src/osgViewer/GraphicsWindowCocoa.mm

@@ -1223,6 +1223,7 @@ bool GraphicsWindowCocoa::realizeImplementation()
     [_view setAutoresizingMask:  (NSViewWidthSizable | NSViewHeightSizable) ];
     [_view setGraphicsWindowCocoa: this];
     [_view setOpenGLContext:_context];
+    [_view setWantsBestResolutionOpenGLSurface: NO];
 
     // enable multitouch
     if (_multiTouchEnabled || (windowData && windowData->isMultiTouchEnabled()))
@@ -1305,7 +1306,6 @@ bool GraphicsWindowCocoa:: makeCurrentImplementation()
 {
     if (_updateContext)
     {
-        [_context update];
         _updateContext = false;
     }