Image2d refactoring & more
- Changed Align class offset-interface. Now there's PointOffset(), PointOffsetCenteredQuad() and RectOffset() (unimplemented)
- Added Screen <-> Viewport point conversion methods to Renderer interface and implementations.
- Renamed ScaleType=>RefScale. Better naming for enums values. Added also fit rectangle scale for image and screen cases. Added also RefScaleToString().
- Renamed Image2d::DrawRel() => Image2d::DrawViewport(). Parameters: changed order, added default values, added option of giving offset vector instead of Align. Supports also Align directly.
  - Image2dSdl2Opengl has working implementation
  - Image2dSdl2 still not implemented. Waiting for the interface to take it's finished shape.
- Added Texture::GetAspectRatio().
- Commented debug output from input