Drawing system update

• Unify drawing systems into one
• Draw directly to screen

Draw world drawables in sorted order

v0.4.0

Add some doc comments

Revise sprite package

• Rename spriteset to spritesheet
• Add sprite.Link
• Add engine.SetupTypes

Fix animations schema

v0.2.1

Fix animations schema

Pre-release for prototyping

• Add generic point, size, and rectangle.
• Combine imageset and imageresource into sprite package.

Pre-release for prototyping

This pre-release will help prototyping a game that is based on it.