Fix improper NSGraphicsContext handling in wxOSXCreateBitmapContextFromNSImage.

Don't leave currentContext set to the temporary context indefinitely,
but restore the previous one when done. It's apparent from the code that
this is how it was meant to be done.

Not doing this can result in strange, insanely hard to debug errors in
completely unrelated places, because OS X (at least < 10.9) reuses a
pool of contexts. For example, this change fixes Quicklook crashes in
file open panel on 10.8.

For detailed explanation, see the lengthy discussion at
https://code.google.com/p/chromium/issues/detail?id=90140

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75708 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775