[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.