Add wxCOMPOSITION_INVALID mode and use it to simplify the code.

Add wxCOMPOSITION_INVALID mode and use it to simplify the code.

Having an invalid element in wxCompositionMode enum allows to directly return
it from TranslateRasterOp() function instead of needing a separate bool
parameter to indicate that a ROP couldn't be translated.

This also incidentally fixes warnings about possibly uninitialized variables
in optimized g++ builds.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68127 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775