* Migration to the `opengl-bindings` gem.
  The `opengl` gem is deprecated since years and no longer maintained.
  `opengl-bindings` is the announced successor.
* Add an alternative location for API documentation: https://larskanis.github.io/fxruby/
  This is because https://rubydoc.info is too often down.
* Fix Canvas::ImageShape and shape selection. #60
  This changes the position the text of TextShape is printed on the canvas