Compilation fix after r75765.

Forgot to backport ResetAlpha() addition which is used by the code of r75765.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75772 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775