* add writeable image support to engine
* add external resources to engine
* update fine to output to a texture
* copy over original piet-gpu test scenes
* put it all in a pretty (resizable!) window
This adds a new dependency on peniko, reintroduces kurbo for geometry (!), removes the now defunct types from piet-scene and updates all the test scenes to use the new types.
Still one flaw, fat lines aren't expanded with strokes in path coarse rasterization. But that's a small visual ding, and can be fixed
That said, there is some really strange stuff going on in tile_alloc. It's using storage to do a uniform broadcast (the result of bump allocation for the workgroup), which is not great at all. It should be using workgroup storage, but on my mac it behaves as if the workgroup barrier is not in place. Investigating.
Starting an experimental port to WGSL shader language, using wgpu to run the examples. As of this commit, it's quite hacky and takes some shortcuts, but does render paths to a grayscale texture.
