contrib: ffmpeg: use -Og in non optimized builds

It's better than -O0 in some cases (like clang on i686)
15 jobs for !20 with llvm-tools/1 in 21 minutes and 18 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build
canceled #408442
amd64 docker
android-arm

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android-arm64

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android-x86

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android-x86_64

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debian

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macos
ios-arm64

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ios-armv7

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macos

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raspbian-arm

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snap

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uwp64-libvlc-llvm

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win32

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win32-llvm

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win64

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passed #408434
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win64-llvm

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