texture: update for rust-image API changes.
Provide multi-version GLSL shaders and default to 3.3 (core).
Use the correct winding order for cube faces, and enable culling.
Basic first-person camera controls.
* WASD for forward/back and strafe left/right.
* Space/shift for flying up/down, ctrl increases horizontal speed.
* Mouse movement controls yaw (horizontal) and pitch (vertical) angles.
Merge pull request #52 from eddyb/cam-controls

Basic first-person camera controls.