Update to FFMPEG_jll 4.3.1 with optional nvidia hardware accelerated encoding

[IanButterworth]

Waiting on JuliaPackaging/Yggdrasil#1384
Will need re-running once that releases

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 2.56%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master       #32      +/-   ##
===========================================
+ Coverage   97.43%   100.00%   +2.56%     
===========================================
  Files           1         1              
  Lines          39        38       -1     
===========================================
  Hits           38        38              
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
src/FFMPEG.jl	100.00% <ø> (+2.56%)	⬆️

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[IanButterworth]

Tests are passing here. FreeBSD appears to be an unrelated CI julia issue (any idea @ararslan ?)

I've tested downstream VideoIO on a MacOS non-gpu, and ubuntu gpu system, and tests pass.
The VideoIO tests don't yet cover the h264_nvenc codec though, so that's experimental, but codec information does get printed via FFMPEG.exe(`-h encoder=h264_nvenc`).

I believe this should be a bump to 0.4.0 given it upgrades ffmpeg from 4.1 to 4.3

@SimonDanisch what do you think? I want to be careful about messing up downstream plotting packages, which I already did twice today already..

[IanButterworth]

The minor version bump should protect downstream packages here, so I'll merge and release

[ararslan]

Ah yeah, we should switch the FreeBSD CI here from Cirrus to Travis. Cirrus FreeBSD images inexplicably stopped working a while ago, luckily just in time for Travis to add support for it.