Support for multiple GL windows with context sharing

[processing-bot]

Created by: codeanticode

Currently, multiple windows can be created from a Processing sketch, but when using the GL renderer, they do not share the GL context. This means, in particular, that GL resources cannot be shared among the windows, which would the default assumption when creating multiple windows in the first place.

JOGL has introduced some new APIs to enable context sharing between multiple GLWindows:

https://jogamp.org/deployment/jogamp-next/javadoc/jogl/javadoc/com/jogamp/opengl/GLSharedContextSetter.html

GL object synchronization seems to be an issue, specially on Mac:

https://jogamp.org/deployment/jogamp-next/javadoc/jogl/javadoc/com/jogamp/opengl/GLSharedContextSetter.html#synchronization

[processing-bot]

Created by: benfry

This checks out with the version I'd been hacking on, so I'm merging #313 for beta 3 so we can shake out any potential issues.

We can keep this open until we're comfortable that it's resolved properly.

[processing-bot]

Created by: benfry

Fully incorporated for 4.0 beta 3.

[processing-bot]

Created by: github-actions[bot]

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