Fixed order of members in IntRect
Extracted sprite rects to named constants.
Added /data_src to .gitignore
Added fancy parallax effect to starfield.
Rename Rigidbody to PhysParticle.
Add (new) Rigidbody class.

Angle/rotation logic will be used in lots of objects, so this class is
introduced to avoid code duplication.
Add vector version of SpriteMatrix::rotate.
Add some named constants to vec2.hpp
Use vectors instead of angles in Rigidbody.

This mostly avoid using sin/cos all the time for rendering, but will
come in handy later too.
Always optimize starfield.cpp.

The heavy use of the RNG was making the code too slow in unoptimized
builds.
Use Rigidbody in Ship.
Switch back to PhysParticle in Bullet.

Bullets don't have angular momentum so most of the things in Rigidbody
went unused.
Added art for debris and shield hit.
Drones can now be killed and spawn Debris.
Tweaked Debris appearance.
Reduced Ship turn speed.
Increased Debris fadeout period.
Add shield hit gfx to Drone.