DXVA2: Improve dimensions guessing and don't crash when we really can't guess

Here a some improvements regarding for the DXVA2 decoder:
 - `coded_width` and `coded_height` are not always set by FFmpeg (apparently, I've opened a ticket for clarifications about this) so try to fallback to `width` and `height`.
 - if we really can't figure out the dimensions, just refuse to decode instead of crashing.