This change introduces the new APIs RunGame, WindowSize and
SetWindowSize. These new APIs hides the notion of 'scale', and is
more flexible with the outside size change. This means that we can
introduce a resizable window.
This change also adds -legacy flag to examples/windowsize. If the
flag is off, the new APIs are used.
This change deprecates these functions since the notion of 'scale'
is deprecated:
* ScreenScale
* ScreenSizeInFullscreen
* SetScreenScale
* SetScreenSize
Fixes#943, #571
Updates #320