Use shared GL context for all FBOCanvases

[hageldave]

As was pointed out here, AWTGLCanvas can use a shared GL context so that GL resources can be shared among Canvases.
Due to a misconception (and poor research) on my part the FBOCanvas features an id that is accessible during render which is used by CharacterAtlas to return the correct atlas for a context.

This should be ditched and instead all FBOCanvases should share the same context.
This also allows for the shaders to be created only once.

[hageldave]

Did some first refactorings on a side branch bcbde6b
but have to wait until gl context sharing is officially supported by lwjgl3-awt.