Add screen-gap core option (#128)

* Add screen-gap core option

* Fix screen-gap with OpenGL
13 jobs for master in 30 minutes and 22 seconds (queued for 139 minutes and 7 seconds)
Status Name Job ID Coverage
  Build Shared
passed android-arm64-v8a #1035284

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passed android-armeabi-v7a #1035283

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passed android-x86 #1035286

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passed android-x86_64 #1035285

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passed libretro-build-ios-arm64 #1035287
macosx-packaging

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passed libretro-build-linux-i686 #1035280

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passed libretro-build-linux-x64 #1035279

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passed libretro-build-osx-arm64 #1035282
macosx

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passed libretro-build-osx-x64 #1035281
macosx

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passed libretro-build-tvos-arm64 #1035288
macosx-packaging

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passed libretro-build-windows-i686 #1035278

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passed libretro-build-windows-x64 #1035277

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  Build Static
passed libretro-build-libnx-aarch64 #1035289

00:03:00