Fix transparency in toolbar buttons when not using comctl32.dll v6.

Old versions of comctl32.dll don't support alpha in the toolbar image list, so
use the masks only for them.

This is a backport of r75785, r75794 and r75923 from trunk.

Closes #2609.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75924 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775