Clear larger area in wxGCDC::Clear().

Clearing a rectangle of size 32000 was not always enough to clear the entire
DC, so use INT_MAX instead -- this is the best we can do with the current
approach.

Closes #13445.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71944 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775