Implement .bmp image loading.

- Implement loading of .bmp images with 1bpp, 2bpp, 4bpp, 8bpp and 24bpp color depth.
- gui.guidemo: Add image viewer functionality.
- rt.color: Add contructor taking one byte for each channel.
- rt.sceneloader: Add filePath property to SceneLoadContext which contains
the path to the currently loaded scene file. Some refactoring.
- rt.texture: Fix file loading not basing relative path on the location of the scene file.