* Allow to parse four element vectors
* Allow colors defined in HTML notation (#rrggbbaa)
in CEL scripts
* Refactor Overlay::Rectangle to an independent class
* Refactor CelestiaCore::OverlayImage to an independent class
* Refactor CelestiaCore::View to an independent class
* Rename Renderer::getScreenSize to getViewport
* Add wrappers for scissors test and MSAA control