Player motion not taken into account when throwing projectiles

The bug

When the player throws a projectile, its motion ignores the player's, causing it to follow an incorrect trajectory.

How to reproduce

Method 1:

1. Get a splash potion in creative mode
2. Run and sprint while throwing it repeatedly
   Expected results: You throw it forward with the same distance as though you were standing still
   Actual results: You catch up with it and it lands directly on you

Method 2:

1. Get a splash potion in creative mode
2. Go up 100 blocks or so and let yourself fall 70
3. Quickly throw the potion forward
   Expected results: It lands soon after you do, same as though you were standing still
   Actual results: It takes several seconds to land, because it started with zero downward motion