gstffmpegenc: Try getting the caps for the declared pixfmt without a context.
PixFmt that are declared in AVCodec.pix_fmts are ones which are 'officially' declared as being supported. We should therefore not have to create a AVCodecContext and open an encoder to know if it's supported or not. Also, doing it this way allows us to better pickup configuration overrides we have in gstffmpegcodecmap for some codecs (like restrictions on width, height, framerate like it's the case for dnxhd). Fixes #575545
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