Correct Color to RGB gamma treatment, fix Checker texutre class.
- Fix Checker texutre class's getTexColor(..) method.
- Fix Color's toRGB32 method by properly adjusting for gamma.
- Add {@safe, @nogc, pure, nothrow and const} attributes to Color's methods (as appropriate) and move some code around.
- Make Ray struct self-contained (remove reference to gfm.math.shapes).
- Move screen clearing in SDL2Gui to the init() method.
- Whitespace fixes to app.d and rt/renderer.d.