Add screen-gap core option (#128)

* Add screen-gap core option

* Fix screen-gap with OpenGL
13 jobs for master in 29 minutes and 22 seconds (queued for 137 minutes and 21 seconds)
Status Name Job ID Coverage
  Build Shared
passed android-arm64-v8a #1038564

00:01:24

passed android-armeabi-v7a #1038563

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passed android-x86 #1038566

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passed android-x86_64 #1038565

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passed libretro-build-ios-arm64 #1038567
macosx-packaging

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passed libretro-build-linux-i686 #1038560

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passed libretro-build-linux-x64 #1038559

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passed libretro-build-osx-arm64 #1038562
macosx

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passed libretro-build-osx-x64 #1038561
macosx

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passed libretro-build-tvos-arm64 #1038568
macosx-packaging

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passed libretro-build-windows-i686 #1038558

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passed libretro-build-windows-x64 #1038557

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  Build Static
passed libretro-build-libnx-aarch64 #1038569

00:02:56