Fix grenades going backwards when the player is moving.

Fix grenades going backwards when the player is moving.

Most weapon bullets have a speed which is much higher than the player
can move, but grenades are relatively slow. Since Bob started moving
faster many versions ago, grenades were actually slower than Bob
himself. Since there was no concept of bullets inheriting the speed of
the entity shooting them, that meant that when Bob was moving forward
and tried to throw a grenade, the grenade would actually move backwards
relative to Bob.

We fix that now by making grenades inherit the velocity of the entity
throwing them. Unfortunately, a player that is on a moving platform is
considered not to have any velocity, even if the platform is moving. In
stead of rewriting everything to track the actual velocity of entities
on platforms, we just check if the grenade throwing entity is on a
platform, and if so add the platform's velocity to the grenade.

With the old starting velocity of grenades plus Bob's own speed,
grenades could actually be thrown very fast. Compensate that by making
the starting velocity of grenades much lower. It might make some puzzles
involving grenades a bit harder.