This patch implements the pad, repeat and reflect extend modes for gradient brushes. Adds a new example demonstrating the functionality.
Also fixes a few bugs:
* Clamps alpha in blend.wgsl for the `blend_compose` function. The `Plus` mode was generating `alpha > 1.0` leading to incorrect rendering.
* Small change to radial gradients in fine.wgsl to reject pixels outside the cone when the circles don't nest. This requires further work to properly extend the cone when one of the radii is not 0.
This fixes an incorrect application of the inverse transform for radial gradients in fine.
Also fixes an edge case in `SceneBuilder` where a brush transform is identical to the path transform leading to a corrupt encoding.
The area bug was found and fixed by @dfrg, and is adapted from #239. I wanted to move it to a separate PR so that one would be more focused on API.
The other bug is currently silent because the two quantities swapped are both 4, but it is triggered when experimenting with tuning for performance.
* Move the vello crate to the root of the crate
* Add warning that README is work in progress
* Add newline in warning
* Move the unlicense into the shader folder
* Fixup wgsl-analyzer include paths
