FFMPEG setup

Contains the following squashed commits from the old branch:

Initial commit

Fix link order

Add option to link against system ffmpeg, disable precompiled binaries for mingw

Update macOS files for XCode 12

Update ffmpeg to 5.1.2

Add mjpeg encoder (#7)

Add mjpeg2jpeg bitstream filter

Add macOS-arm64 builds

Add macOS arm64 to CMakeLists

Add arm64-osx-release to build.yml

dummy commit to trigger workflow

Properly split macOS builds in build.yml

Try to fix actions uploads

Rename the builds

Fix build.yml

Fix typo in build.yml

Amend CMakeLists using the correct check

ci: change upload path to directly upload libraries

ci: build and use universal binary on macOS

ci: set macOS deployment target to macOS 11.0

ci: build FFMPEG prebuilt for each OS/architecture

Add linux arm64 prebuilt.
Add both x86_64 and arm64 prebuilts for macOS.

ci: create release for pre-built FFMPEG

cmake: download and use only necessary prebuilt FFMPEG

Add arm64 windows prebuilt

Co-authored-by: Seungyun Lee <khora.lee.0@gmail.com>
Co-authored-by: shinra-electric <50119606+shinra-electric@users.noreply.github.com>
Co-authored-by: SaturnSky <ikadro@gmail.com>

Add arm64 windows prebuilt