Fix compilation for external projects.

The mixed-in call to createRenderingCode() was invalid because createRenderingCode is private in a foreign module - making it accessibly by putting it into the Context interface.
Add optional renderEmbd() functions for convenient procedural style rendering inside of vibe.d projects.
Add usage example.
Merge pull request #1 from s-ludwig/master

Compile fix for external projects + vibe.d convenience functions.