We render pack man at twice the size that we want on screen,
which is the native resolution of the asset file.
The maze is upscaled.
We then project everything onto a view which is applies
the scale factor.
This patches also dectect the appropriate scale on OSX.
This required a bit of objective C (scaling.mm),
but students should not look at this file and there
is a comment in that direction.
Scalling.cpp provides the default implementation
for other platforms.
This simplifies the rendering code and remove all(!) pointers.
There is a number of unresolved issues
* Windows build ?
* position delta computation: migrating to SFML seems to have modified
the frame rate.
