This commit expands Bevy's existing tonemapping feature to a complete
set of filmic color grading tools, matching those of engines like Unity,
Unreal, and Godot. The following features are supported:

* White point adjustment. This is inspired by Unity's implementation of
  the feature, but simplified and optimized. *Temperature* and *tint*
  control the adjustments to the *x* and *y* chromaticity values of [CIE
  1931]. Following Unity, the adjustments are made relative to the [D65
  standard illuminant] in the [LMS color space].

* Hue rotation. This simply converts the RGB value to [HSV], alters the
  hue, and converts back.

* Color correction. This allows the *gamma*, *gain*, and *lift* values
  to be adjusted according to the standard [ASC CDL combined function].

* Separate color correction for shadows, midtones, and highlights.
  Blender's source code was used as a reference for the implementation
  of this. The midtone ranges can be adjusted by the user. To avoid
  abrupt color changes, a small crossfade is used between the different
  sections of the image, again following Blender's formulas.

A new example, `color_grading`, has been added, offering a GUI to change
all the color grading settings. It uses the same test scene as the
existing `tonemapping` example, which has been factored out into a
shared glTF scene.

[CIE 1931]: https://en.wikipedia.org/wiki/CIE_1931_color_space

[D65 standard illuminant]: https://en.wikipedia.org/wiki/Standard_illuminant#Illuminant_series_D

[LMS color space]: https://en.wikipedia.org/wiki/LMS_color_space

[HSV]: https://en.wikipedia.org/wiki/HSL_and_HSV

[ASC CDL combined function]: https://en.wikipedia.org/wiki/ASC_CDL#Combined_Function