Remove references to private property _entities from collider.spec.js
Add getter method for renderer.backgroundColor

Eventually, the property will no longer be directly accessible,
but set in the semi-private space instead.
Remove property renderer.backgroundColor

The value is now retrieved from the 'backgroundColor' property
of the space in the getter.
Remove property renderer.game

I introduced a local variable set from the 'game' property
of the space in update().

I also introduced a local variable gameEntities in update().
Refactoring: extract function configureCanvas() from Renderer()
Remove property renderer.viewSize
Remove reference to renderer.ctx in unit tests

I created a local variable to hold the value instead.
Replace property renderer.ctx with "canvasContext" in space
Remove unused local variables in Renderer() constructor
Add missing declaration of local variable (var)
Remove property renderer.viewCenterPos
Add space character after colon in object litteral
Remove unused variable interval at closure level of Ticker()
Revert "Remove unused variable interval at closure level of Ticker()"

The variable is actually used in window.requestAnimationFrame().

This reverts commit 255efe17629a5292f579dad7974ded83936e3cce.
Move variable interval close to its actual use, rename to INTERVAL
Remove property entities.game
Remove property entity._entities

The current list of entities is now set in the property
"gameEntities" of the semi-private space.
Move comment above line to break long line
Refactoring: extract 4 functions from Inputter() constructor

I extracted in turn:
* configureCanvasFocus() -> suppressScrolling() -> preventScrolling()
* configureKeyListeners()

I also extracted onKeyDown(= and onKeyUp() within Inputter().
Remove properties inputter._keyDownState and inputter._keyPressedState