Parallax and z-layered imgs working, some bugs

Code has been well-cleaned and commented. More detailed explanations can
be found within.
Camera, player, z-layered imgs, and parallax all work as intended. All
canvases are now created within the dart code, not the html.
Two glaring issues: Images DO NOT draw at the appropriate y-value.
Foreground and background images position well, middle layers do not.
The game occasionally will not load images when run; trying to run again
generally fixes.