Add support for other LWJGL- and OpenGL versions

[javagl]

Based on the discussion in #11 (comment) there are two tasks that would help to increase the number of supported platforms:

1. Adding support for LWJGL3. As a replacement of the AWTGLCanvas that was removed in LWJGL3, the https://github.com/httpdigest/lwjgl3-awt could be used. A pressing reasons for an update to LWJGL3 (or rather: for an additional viewer implementation based on LWJGL3) could be when someone wants to use the viewer in his own "standalone" progam that is already based on LWJGL3
2. Retrofit the existing viewer implementations so that they only need OpenGL 2.0. This should be easy, because except for vertex array objects, all the functionalities that are currently used have already been available in OpenGL 2.0.

[javagl]

I created a basic implementation of a viewer based on LWJGL 3 and lwjgl3-awt. It is not yet committed, but basically seems to work. It may be added with one of the next (preview) releases.

[javagl]

Combining LWJGL2 and LWJGL3 in one package may turn out to be far more difficult than it should be. The native library has the same name in both cases. Both are called lwjgl.dll....

[javagl]

Based on some disucssion in LWJGL/lwjgl3#269 (comment) , the support for LWJGL3 is deferred for a while. I'll probably add the current state later, together with a "mini demo" to show that it basically works (on Windows), but not yet include it as an alternative rendering backend for the next jgltf-browser release.