Make debug actually print the error when ffmpeg crashes, also fix some channel mapping stuff

[mjc]

ffmpeg has this open issue about mapping channels correctly with libopus.

[libopus @ 0x56555780a0c0] Invalid channel layout 5.1(side) for specified mapping family -1.
Error initializing output stream 0:1 -- Error while opening encoder for output stream #0:1 - maybe incorrect parameters such as bit_rate, rate, width or height

I suspect the correct fix for this is to have our own mappings for these but for now this PR at least let the encode continue and I figured I would open the discussion so I can fix this PR to do whatever is decided on a weekend.

ffmpeg channel maps (from ffmpeg -layouts):
mono: FC
stereo: FL+FR
2.1: FL+FR+LFE
3.0: FL+FR+FC
3.0(back): FL+FR+BC
4.0: FL+FR+FC+BC
quad: FL+FR+BL+BR
quad(side): FL+FR+SL+SR
3.1: FL+FR+FC+LFE
5.0: FL+FR+FC+BL+BR
5.0(side): FL+FR+FC+SL+SR
5.1: FL+FR+FC+LFE+BL+BR
5.1(side): FL+FR+FC+LFE+SL+SR
6.0: FL+FR+FC+BC+SL+SR
6.0(front): FL+FR+FLC+FRC+SL+SR
hexagonal: FL+FR+FC+BL+BR+BC
6.1: FL+FR+FC+LFE+BC+SL+SR
6.1(back): FL+FR+FC+LFE+BL+BR+BC
6.1(front): FL+FR+LFE+FLC+FRC+SL+SR
7.0: FL+FR+FC+BL+BR+SL+SR
7.0(front): FL+FR+FC+FLC+FRC+SL+SR
7.1: FL+FR+FC+LFE+BL+BR+SL+SR
7.1(wide): FL+FR+FC+LFE+BL+BR+FLC+FRC
7.1(wide-side): FL+FR+FC+LFE+FLC+FRC+SL+SR

opus follows vorbis channel ordering
stereo: L+R
3ch: L+C+R
4ch: FL+FR+RL+RR
5ch: FL+C+FR+RL+RR
5.1: FL+C+FR+RL+RR+LFE
6.1: FL+C+FR+SL+SR+RC+LFE
7.1: FL+C+FR+SL+SR+RL+RR+LFE

It looks like just overriding 5.1(side) to 5.1 is fine but I'm not so sure about most of the other cases

[FallingSnow]

Thanks for the pull, poked my to check out the other pull requests.

I think pull #127 by @upekshapriya, specifically https://github.com/FallingSnow/h265ize/pull/127/files#diff-b5aeacf0b0e89c4ad9316070981779a4R819, will fix the problem sufficiently, don't you? Let's see what happens with his pull first.

I will take the debug fix though! Thank you.

[mjc]

It may. I'll make a new PR with just the debug commit.