Fix crash when drawing bitmaps with mask in wxGTK with GTK+ < 2.20.

r73386 introduced a crash in wxGTK drawing code when using old GTK+ versions
by releasing the mask pixmap prematurely. Apparently GTK+ itself was fixed at
some time in 2.19.x time frame but work around the bug for the sake of older
GTK+ versions in wxGTK itself by simply releasing the pixmap at the end of
DoDrawBitmap(), as pre-r73386 code did.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73549 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775