JRuby Support

[danini-the-panini]

Work around some issues which prevented Mittsu from running on any version of JRuby.

1. Fiddle does not automatically convert function arguments to pointers unless they have a to_ptr method. Fiddle::Closure does not have a to_ptr method, and therefore cannot be passed as an argument (e.g. GLFW callbacks). Manually converting them to a pointer works around this issue.
2. Similarly, Ruby integers do not have a to_ptr method, And must therefore be manually converted to pointers before being passed as function arguments.
3. nil, however, is automatically converted to a nullptr in JRuby (and thus works as a suitable substitute for 0)

TODO:

• [x] Get blank scene example running in JRuby
• [x] Get geometry examples running in JRuby
• [ ] Some of the more complex examples (e.g. torus knot example) quit with no output
• [ ] The tests also quit with a non-zero exit code with no output (possibly same issue as above)
• [ ] Add JRuby to github action test matrix
• [ ] print_tree just prints question marks instead of the special drawing chars (could be an encoding issue? could be a windows-only issue?)

[headius]

We are willing (and eager) to make improvements to JRuby's fiddle impl! Some of these seem like things we should fix at the source. Great work!