user32: glclock: maximizing app window leaves its original area frozen

To replicate this issue, start the GLClock app (glclock.exe from link provided). It will spawn a small window with a 3d, rotating clock. Now, maximize this window. As you can see from screenshot attached, the area where app window was initially, is not being updated. When you click to set app back to its original window, it is still not updated. Only when you drag and move window, its gets back to normal. This issue has been regress-tested to r54638, with 54637 working correctly. Assigning to commiter for review.