Add a basic lines-based debug-renderer

[sebcrozet]

This adds a basic debug-renderer to Rapier (hidden behind a new debug-render cargo feature).
This exposes a DebugRenderPipeline that can render rigid-bodies, colliders, and joints as long as the user provides a way to render lines. The main objective here is to provide a very easy ways for users to see what the physics engine sees.

The line renderer interface is fairly straightforward:

pub trait DebugRenderBackend {
    fn draw_line(
        &mut self,
        object: DebugRenderObject,
        a: Point<Real>,
        b: Point<Real>,
        color: [f32; 4],
    );

The most naive backend could simply draw the segment [a, b] using the given color. More elaborate backend way want to color things based on the way the object is rendered.

The color given to this method is computed from a style which can be modified by the user:

#[derive(Clone, Debug, PartialEq)]
pub struct DebugRenderStyle {
    pub subdivisions: u32,
    pub collider_dynamic_color: DebugColor,
    pub collider_fixed_color: DebugColor,
    pub collider_kinematic_color: DebugColor,
    pub collider_parentless_color: DebugColor,
    pub impulse_joint_anchor_color: DebugColor,
    pub impulse_joint_separation_color: DebugColor,
    pub multibody_joint_anchor_color: DebugColor,
    pub multibody_joint_separation_color: DebugColor,
    pub sleep_color_multiplier: [f32; 4],
    pub rigid_body_axes_length: f32,
}

Here is an example of this debug-rendering: