Movie + InTheaters: Retina devices fix.

These fix does three things:
- is_retina is now deprecated so we're switching to DDG.is3x and DDG.is2x.
- Using is_retina fails on IE 10 and lower. Turns out undefined variables are invalid, but window.is_retina is valid.
- ~~The posters are blurry on iPhone 6 Plus (3x screens) so we use the original image.~~