The value of wxColour::Alpha() was assigned to a temporary variable which
wasn't used afterwards and Mac OS g++ warned about it.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61984 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
These methods allow to operate with all 3 or 4 colour channels at once.
Add their implementation, documentation and a unit test for wxColour
exercising them.
Closes#9918.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61976 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
