Force the video display to LTR layout direction

Mirroring the video is not useful or desirable.

See #1354.
Mirror toolbar button images for RTL locales

Some of the toolbar buttons are seriously wrong when mirrored and there
doesn't seem to be a way to disable mirroring for the images while using
RTL layout for the toolbar itself, so double-mirror them so that they
end up with their original appearance.

Ideally the button images would be localizable as some of them actually
should be mirrored, but that's probably not worth the work involved.

See #1354.
Fix RTL painting of the subtitles grid

wx comes very close to just making it work automatically, but it doesn't
translate the origin to the top-right corner and wxBufferedPaintDC
doesn't handle RTL at all, so reimplement it with some hacks.

Closes #1354.
Update wx