Avoid problems with conflicting UINT16 definitions in Cygwin headers.

UINT16 is incorrectly defined inside Gdiplus namespace in w32api gdiplus.h
used by Cygwin and MinGW resulting in compilation errors about ambiguous
symbols.

Fix this by forcefully redefining UINT16 in our own code.

Closes #13113.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67970 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775