Cleaned up getRectsBeingDrawn:count: related code.

Cleaned up getRectsBeingDrawn:count: related code.
* Removed the exception handler; it's not needed anymore (thanks Ryan).
* Split the if statement and then case into two lines instead of one.
* Took some of the suggestions from the cocoa-dev mailing list:
- Changed code to use normal messaging syntax.
- Provided a category interface to silence the compiler warning on 10.2.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30067 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775