Port 1.14.3 code to OSX

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For Mac OSX support, a native library is needed (or some functions, if there are Java bindings to the relevant APIs) that takes a ByteBuffer of RGBA pixel data, and copies it to the clipboard. I don't have a Mac to build or test this code on, so I'll need other people to contribute.

Resources that may help:

• https://github.com/LWJGL/lwjgl3/blob/master/modules/lwjgl/core/src/generated/java/org/lwjgl/system/dyncall/DynCall.java
• https://javadoc.lwjgl.org/org/lwjgl/system/macosx/ObjCRuntime.html
• https://gist.github.com/mortenjust/6ee99ad122e5348b8f9c
• https://developer.apple.com/documentation/appkit/nspasteboard

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Code contributed and tested thanks to @juliand665!