Add random trajectory spread to throwable projectiles

[HashimTheArab]

In vanilla, all throwable projectiles (snowballs, eggs, ender pearls, wind charges) have a small random offset applied to their trajectory on each axis using a triangular distribution (random.triangle(0, 0.0172275)). Dragonfly currently fires all throwables in an exact straight line with no spread.

The vanilla algorithm (per the wiki):

1. Get the unit facing vector of the player (with a per-item pitch offset)
2. For each axis, add a random value in [-0.0172275, 0.0172275] using a triangular distribution
3. Scale the result by the item's POWER
   4. Add the player's velocity (Y component unaffected if the player is on ground (java only)

Affected items: Snowball, Egg, EnderPearl, WindCharge, and Bow/Crossbow

This would look like this

const spread = 0.0172275
vel = vel.Add(mgl64.Vec3{
	spread * (rand.Float64() - rand.Float64()),
	spread * (rand.Float64() - rand.Float64()),
	spread * (rand.Float64() - rand.Float64()),
})