Add `flex` scaling mode
Add `flex` scaling mode

- closes #656

gopalindians/melonJS,gopalindians/melonJS,TukekeSoft/melonJS,melonjs/melonJS,TukekeSoft/melonJS,melonjs/melonJS,rlugojr/melonJS,TukekeSoft/melonJS,srbala/melonJS,rlugojr/melonJS,gopalindians/melonJS,rlugojr/melonJS,srbala/melonJS,srbala/melonJS