Support deep custom rendering logic

[qq1053831109]

I want to create a 3D model batch viewer。I have one libgdx application.He needs to render many Canvas。 in one libgdx Thread。now openglfx use one Thread for one Canvas。 not one Thread one libgdx for many Canvas.

[husker-dev]

Sorry for the silence, I have some things to do at work.
I see some solutions to this issue, but I will check them later when I have free time