Support framebuffer plugins

[tmcw]

My understanding of framebuffers is pretty tenuous, but the ideal is that we could

1. Render each tile to a buffer
2. Pass that buffer through configurable shaders

This would help with things like full-view blurring, color-blindness simulation, and photoshop-style effects. The hope is that since at this point you're dealing with a generic image rather than any specific data, it's a place where configurable plugins could be applied to any map.

Over the weekend, I sketched this out but was very dumb about it and probably made many mistakes.

[lucaswoj]

This is cool! Do you think this is something we should try to include in an upcoming release?

Any thoughts about how this would work with tile-less rendering @ansis?

[lucaswoj]

💭 I wonder how this jivves with #281

[tmcw]

Closing in favor of #281