joints example not properly working

[ThierryBerger]

TL;DR: fix in rapier: dimforge/rapier#660

description

steps:

• cargo run --example joints2

Expected render

Actual render (as of 56f9870)

• Same behaviour observed for joints3

Analysis

introduced in #525 (5fb8a4e)

Position matrix for the dynamic entities contain infinity values 🤔

[ThierryBerger]

Will be fixed with rapier 0.21, keeping opened until we update.