This commit implements actual rendering for the new piet-scene crate.
In particular, it adds a new EncodedSceneRef type in piet_gpu::encoder to store references to the raw stream data. This is used as a proxy for both PietGpuRenderContext and piet_scene::scene::Scene to feed data into the renderer.
Much of this is still hacky for expedience and because the intention is to be a transitional state that maintains both interfaces until we can move to the new scene API and add a wrapper for the traditional piet API.
The general structure and relationships between these crates need further discussion.
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).
This patch switches to a variable size encoding of draw objects.
In addition to the CPU-side scene encoding, it changes the representation of intermediate per draw object state from the `Annotated` struct to a variable "info" encoding. In addition, the bounding boxes are moved to a separate array (for a more "structure of "arrays" approach). Data that's unchanged from the scene encoding is not copied. Rather, downstream stages can access the data from the scene buffer (reducing allocation and copying).
Prefix sums, computed in `DrawMonoid` track the offset of both scene and intermediate data. The tags for the CPU-side encoding have been split into their own stream (again a change from AoS to SoA style).
This is not necessarily the final form. There's some stuff (including at least one piet-gpu-derive type) that can be deleted. In addition, the linewidth field should probably move from the info to path-specific. Also, the 1:1 correspondence between draw object and path has not yet been broken.
Closes#152
* 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
This PR reworks the clip implementation. The highlight is that clip bounding box accounting is now done on GPU rather than CPU. The clip mask is also rasterized on EndClip rather than BeginClip, which decreases memory traffic needed for the clip stack.
This is a pretty good working state, but not all cleanup has been applied. An important next step is to remove the CPU clip accounting (it is computed and encoded, but that result is not used). Another step is to remove the Annotated structure entirely.
Fixes#88. Also relevant to #119
Make max workgroup size 256 and respect LG_WG_FACTOR.
Because the monoid scans only support a height of 2, this will reduce
the maximum scene complexity we can render. But it also increases
compatibility. Supporting larger scans is a TODO.
