RGB color values are not clamped to the interval [0.0, 1.0]

Not only does kiss3d permit RGB values that are not within the interval [0.0, 1.0], but values outside this interval actually display as different colors.

For instance, take the example code that kiss3d provides, which displays a red, rotating cube, and replace the line
```rust
c.set_color(1.0, 0.0, 0.0);
```
with
```rust
c.set_color(2.0, 0.0, 0.0);
```
It compiles (which it arguably shouldn't) _and_ it actually renders as a different color, which is much more problematic. This means that if one sticks to values between 0.0 and 1.0, they cannot display the full color gamut, which can be a nontrivial handicap.

Likewise, replacing the aforementioned line with
```rust
c.set_color(-2.0, 0.0, 0.0);
```
actually renders a dark aquamarine or turquoise color, rather than black as I would expect (I would actually expect it to not compile at all, but that's beside the point).

The issue names the set_color call and the rotating-cube example, but no source files or tests. Trace set_color from that example through the rendering path and check existing color-related tests. The expected handling of values outside [0.0, 1.0] needs maintainer clarification; done should include agreed behavior and regression coverage.

Written by the indexing model from the issue text.