Allows server owners to use 1.8 (and prior)'s projectile behavior
(ignored shooter's velocity when calculating projectile's velocity).
This patch adds an option "disable relative projectile velocity", which, when
enabled, will cause projectiles to ignore the shooter's current velocity,
like they did in Minecraft 1.8 and prior.
If a player is falling, for example, their shooting range will be drastically
reduced, as a downwards velocity is applied to the projectile. This prevents
players from saving themselves from falling off floating islands, for example,
as a thrown ender pearl will not make it back to the island, while it would
have in 1.8.
While this could easily be done with plugins, too, there are multiple problems:
1) If multiple plugins cancel the velocity by subtracting the shooter's velocity
from the projectile's velocity, the projectile's velocity would be different.
As there's no way to detect whether the projectile's velocity has already been
adjusted to ignore the player's velocity, plugins can't not do it if it's not
necessary.
2) I've noticed some inconsistencies, e.g. weird velocity when shooting while
using an elytra. Checking for those inconsistencies is possible, but not as
efficient as just not applying the velocity in the first place.
3) Solutions for 1) and especially 2) might not be future-proof, while this
server-internal fix makes this change future-proof.
