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 #53, only the whole map #60)
• ~~would be nice to easily change between different render methods~~, done with 65427ba044404be1aeac931edd363ea853322faf