Adds support for image buffers and sampling

[lnihlen]

Fixes #8.

[lnihlen]

This is turning into a never-ending gobstopper of a PR. Remaining work:

• [x] Finish parameterized images support
• [x] Offscreen and fixed framerate stagemanager integration
• [ ] add image rendering tests for constant images and parameterized images
• [ ] add a few different sampler image tests