This patch adds radial gradients, including both the piet API and some
new methods specifically to support COLRv1, including the ability to
transform the gradient separately from the path.
We always do BeginClip/EndClip if it's a solid tile and the blend mode
is not default.
Also fix missing entry in pipeline layout (affects Vulkan but not Metal).
* Add blend and composition mode enums to API
* Mirror these in the shaders
* Add new public blend function to PietGpuRenderContext that mirrors clip
* Plumb the modes through the pipeline from scene to kernel4
Make the scene dependent on timing.
This commit patches the HAL to reuse command buffers; this works well on
Vulkan and prevents a leak, but breaks the other back-ends. That will
require a solution, possibly including plumbing up the resource lifetime
responsibilities to the client.
Other things might be hacky as well.
