Move to crates.io?

[emilio]

First step, avoid git dependencies:

• Move geom to crates.io?
• Auto-generate bindings with gl-rs
• Probably will need to create a branch to publish and delete ther the layers dependency, which is very servo-specific.

Later:

• Use semver
• Rename crate to offscreen_gl
• Rename repo to offscreen-gl-rs

[emilio]

This is done