This removes the need to convert the parameter to copyRectToScreen to
"const byte *", which is commonly used in games, which use Graphics::Surface
to store their graphics data.
All uses of the old target scale API actually wanted to disallow scaling of
the mouse cursor. This commit adapts our API to this and thus simplifies
backend implementations.
Some backends, most notable the Wii and Android, did some implementation of
the cursor target scale, which I didn't adapt yet. I added a TODO for the
porters there.
The errors fixed are inaccessible base class when trying to set the
_graphicsManager and a pure virtual function when trying to create
the NullGraphicsManager instance.
