Bullet mass

[cxong]

Add a mass property to bullets. This will only affect pushback.

Currently pushback is calculated as power * velocity. With mass, this is modified as mass * velocity. For backwards compatibility, mass should be the same as power if not specified.

Usually having pushback affected by power makes sense as the player "feels" how much damage a bullet deals. However decoupling power from pushback means:

• We can have low-pushback weapons where it makes sense, like the rapid-firing but low damage pulse rifle, or flame weapons
• We can have high-pushback weapons, like repulsion weapons, or explosion effects
• The way friction works means that the pushback actually seems wrong for explosions. The C-Dogs explosion "clouds" deal small amounts of damage but over time, which results in very little pushback. Realistic explosions should do a large amount of damage over a short amount of time, with large pushback but low damage for targets further from the epicentre.

[cxong]

Need to update the wiki too, maybe add a negative mass example weapon in techdemo