Scaling cleanup

Status

generally the strategy for creating the initial window should be like this (in all ports):

1. first the optimal size of the canvas is calculated
2. then, the window is created of the right size
3. NO resize message is sent to the window at startup, since the window is already of the right size.
   1. Instead, a resize message is sent to the window only when there's a user action such as enabling double-size, dragging window edges etc

ports that work like this: SDL

Right now 3 sizing models are supported

1. the canvas size is calculated, then the window fits the canvas. Used by most ports when HW scaling is off
2. the canvas size is calculated, the window has an arbitrary width and height, the image is streched/squeezed. Used by most ports when HW scaling is on
   1. the canvas size is calculated, the window's aspect ratio fits the canvas', the window has an arbitrary width or height
3. the canvas size is calculated, the window has an arbitrary width and height, a black border is added to the image if the window is too big or the image is cropped smartly if the window is too small. (was used by Xaw)

TODO: The third one is really ugly (and it is the only reason why we have the unnecessary complication of video-viewport) and should be removed