Perform context scaling according to devicePixelRatio when setting fullscreen

[anderoonies]

Resolves #115
Resolves #100

Summary

setFullscreen and setSize now both perform rounding and devicePixelRatio scaling through the _resize method in the Graphics manager. This fixes issues where setFullscreen would resize based on devicePixelRatio but not scale the context up.

Changes:

• Create _resize method for Graphics manager that performs logic for temporary canvas (for flickering) and scaling
• Use _resize in both setSize and setFullscreen
• Create example for setFullscreen

Checklist

• npm test passes
• Unit tests are included / updated
• Documentation has been updated where relevant