refactor/improve render code

[atomicptr]

The render code isn't very well written:

• While writing openfl-tiled-flixel I couldn't reuse any render code
• The Flash copyPixels implementation has tons of code duplications from the Tilesheet implementation
• The code isn't really flexible (render once, do nothing)
• you can only draw the whole map (no seperate layer drawing Add ability to draw seperate layers #53, only the whole map add ability to only draw a certain part of the map #60)
• would be nice to easily change between different render methods, done with 65427ba