ScaleManager: more updates, fixes (eg. SHOW_ALL)

- NOTE: Changing `game.width/game.height` via user code was always
  problematic. This commit updates the documentation for such members as
  read-only. The only supported way to change the GAME SIZE after it is
  created is to use `ScaleManager#setGameDimensions`, which has been
  added.

- The GAME SIZE will be reset to the initial (or as set by
  `setGameDimensions`) values upon changing the scale mode or
  entering/leaving full screen. This may be a breaking from 2.1.2 (but
  many mode changes acted oddly prior).

- SHOW_ALL will now EXPAND it's parent container if it can. As per
  @tjkopena 's notes, this should more closely represented the expected
  behavior.
  - SHOW_ALL will first try to expand by the OVERFLOW AXIS and then
    attempt to resize to fit into the possibly larger area; use the
    parent's max-height/max-width properties to limit how far SHOW_ALL can
    expand.
  - RE-BREAKING: This changes the behavior from 2.1.4 and makes it more like
    2.1.3, with fixes.

- As per previous commit the ScaleManager _owns_ the margins and size of
  the GAME CANVAS. To control the dimensions of the GAME CANVAS, use the min/max
  height/width of the parent. Setting padding on the parent is _NOT_
  supported.

- Fixes various issues with switching between Scale Modes