Cocoa plugin sends expose events for windows with invalid geometry

To reproduce: run Qt Creator with DYLD_IMAGE_SUFFIX=_debug and switch to the "analyzer" tab.

The warning was added as part of 99480d5420c0beea6771be582c039b550a4461f5 to qtdeclarative to work around that the platform plugin sends expose events for windows with invalid dimensions and for windows with valid dimensions which are outside the viewport of the screen. Both will cause problems. Once the root of the problem has been resolved, we can also revert the rather ugly workaround in the render loop.