Add screen-gap core option (#128)

* Add screen-gap core option

* Fix screen-gap with OpenGL
13 jobs for master in 31 minutes and 50 seconds (queued for 143 minutes and 31 seconds)
Status Name Job ID Coverage
  Build Shared
passed android-arm64-v8a #1032082

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passed android-armeabi-v7a #1032081

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passed android-x86 #1032084

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passed android-x86_64 #1032083

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passed libretro-build-ios-arm64 #1032085
macosx-packaging

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passed libretro-build-linux-i686 #1032078

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passed libretro-build-linux-x64 #1032077

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passed libretro-build-osx-arm64 #1032080
macosx

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passed libretro-build-osx-x64 #1032079
macosx

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passed libretro-build-tvos-arm64 #1032086
macosx-packaging

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passed libretro-build-windows-i686 #1032076

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passed libretro-build-windows-x64 #1032075

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  Build Static
passed libretro-build-libnx-aarch64 #1032087

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