[cosmetic] Fix variable name to match name declared in header and parent constructor
[rbp/omxplayer] do not reopen the resample context for each frame.

I think this doesn't tend to happen on Pi now, as AV_SAMPLE_FMT_S16 and AV_SAMPLE_FMT_FLTP bypass the resampler,
but best to fix it if some obscure codec returns something else.

See #2873
Merge pull request #2881 from popcornmix/avoid_resample_init

[rbp/omxplayer] do not reopen the resample context for each frame.
Added CFileItem::FillInMimeType() and make GetMimeType() respect const qualifiers.

Before, the MIME type would only be resolved if GetMimeType(true) was called, which is potentially blocking. Now MIME types will be resolved from the file's extension without possibly blocking the thread. See doxy of CFileItem::FillInMimeType() for more info.