Rewrite ConvertToGreyscale() and ConvertToDisabled()
They do almost the same thing, so use almost the same code.
Fixes ConvertToDisabled() modifying its own data.
Closes #15389


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74625 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775