Build incremental unit tests

[kirklange]

Feature Rationale
This will be useful for when trying to port projects to platforms/frameworks such mobile and web/emscripten.

Proposed Solution
Have at least the following tests:

• "Hello World" flat, colored window
• Image rendering
• Font rendering
• Grabbing user input
• Toggle-able full screen test
• Simple ball and gravity interactive demo, piecing all the above together

Lastly, investigate how to integrate these with codecov.io/