avutil/hwcontext_mediacodec: fix backward compatibility

AVMediaCodecDeviceContext without surface or native_window is
useless, it shouldn't be created at all. Such dummy AVHWDeviceContext
is allowed before, and it's used by mpv player. Creating a ANativeWindow
automatically breaks such usecases.

So disable creating a ANativeWindow by default. It can be enabled
via the create_window flag, or by set the AVDictionary of
av_hwdevice_ctx_create(). The downside is that

ffmpeg -hwaccel mediacodec -i input.mp4 \
	-c:a copy -c:v hevc_mediacodec output.mp4

use ByteBuffer mode which isn't as efficient as before. The upside
is libavfilter works now, which should be less surprise.

To enable create_window on ffmpeg command line, use
ffmpeg -hwaccel mediacodec \
	-init_hw_device mediacodec=mediacodec,create_window=1 \
	-i input.mp4 -c:a copy -c:v hevc_mediacodec output.mp4

Users should know what it is to enable create_window. It should
be OK to take sometime to figure out the option. And there are comments
inside hwcontext_mediacodec.h to help user figure it out.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

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