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Eliminate usage of internal FFmpeg headers #931

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This eliminates the use of FFmpeg's private headers. There are still other modifications to FFmpeg's public headers preventing use of an unmodified FFmpeg; however, this works towards that goal.

I performed a few tests, but I am not aware of any HTTP streams to test with.

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Instead of the internal FFmpeg header compat/cuda/dynlink_loader.h.  This leaves
only one internal FFmpeg header used by MythTV.

Per the comment on win32_dlopen() in FFmpeg/compat/w32dlfcn.h, Windows will now
additionally search the current directory for nvcuda.dll and nvcuvid.dll, which
FFmpeg considers less secure.

This is an atomic block and the order must be preserved:
 #include "libavutil/log.h"
 #define FFNV_LOG_FUNC(logctx, msg, ...) av_log(logctx, AV_LOG_ERROR, msg,  __VA_ARGS__)
 #define FFNV_DEBUG_LOG_FUNC(logctx, msg, ...) av_log(logctx, AV_LOG_DEBUG, msg,  __VA_ARGS__)
 #include <ffnvcodec/dynlink_loader.h>

 #include "libavutil/hwcontext_cuda.h"
must come after
 #include <ffnvcodec/dynlink_cuda.h>
which is also transitively included by #include <ffnvcodec/dynlink_loader.h>
This reverts commit 5f9b27c.

MythTV no longer uses that file, so remove the modification.
…h (part 1)

Since we don't need the full buffer, we could use avio_read_partial() instead.
…h (part 3)

I don't know if any of this is necessary since FFmpeg's avio API is already
buffered.  However, I did not investigate using avio directly with avio_open()
nor did I investigate what our buffer actually does.

I'm not sure why MythAVFormatBuffer::Seek() with Whence = SEEK_END did not ever
actually seek.

libavformat/avio.h says this about read_packet:
"
A function for refilling the buffer, may be NULL.
For stream protocols, must never return 0 but rather
a proper AVERROR code.
"

So I'm not sure the first return of 0 was correct.  AVERROR(EINVAL) is a more
useful return value for all of the functions, instead of 0.
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Now part of #950

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