Plane primitive: name and use

[fabienrohrer]

The Plane primitive has been introduced mainly to support the corresponding ODE collision primitive. In order render well on screen and for OpenGL limitations on infinite objects, we introduced the size field to limit it. Then we used this feature at many place as a visual/bounded rectangle, without thinking that it's not its original purpose, producing some naming weirdness: "Plane" is not a good name in this case, "Rectangle" or "Quad" would be better.

The purpose of this issue is first to elaborate strategies to fix this.