Don't keep using the old wxBitmap data after modifying it directly in wxGTK.

Drawing on a wxBitmap via wxMemoryDC modified its pixmap representation in
wxGTK and even if its pixbuf representation was later changed using direct
access to the bitmap pixels, the out of date pixmap continued to be used,
creating the illusion of direct access being completely ignored.

Fix this by purging the old pixmap representation when locking the bitmap for
raw access.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75518 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775