Fix simulated toggle in OS X wxToolBar wiht HiDPI bitmaps

Use scaled size and scale factor when creating wxMemoryDC so that the
modified output bitmap has same physical dimensions as the source one.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78090 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775