Rendering issues when using OpenGL and unified toolbar on Mac OS X

When using QGLWidget in a QMainWindow that has unifiedTitleAndToolBarOnMac enabled, rendering issues occur on this unified tool bar.

The attached screenshot shows how the text of the tool button is drawn in an ugly way, and how the background color doesn't look right and shows a white area as a result of resizing the window.

Two more screenshots have been added, showing that the problem does not occur when either unifiedTitleAndToolBarOnMac isn't used or a QGLWidget isn't created (default white central widget).

The example application has been attached as well.

Problem located in my application (Tiled) after upgrading from Qt 4.8. Report with screenshot can be found at https://github.com/bjorn/tiled/issues/529#issuecomment-43900019