WGPU graphics display

[jazzfool]

A graphics display implementation based on WGPU and Lyon. This should outperform the Skia back-end simply due to the smaller scope and the luxury of retained-mode input.

Some general ideas to be implemented:

• 1 command group = 1 VBO + 1 IBO + possibly 1 bind group. Implemented, but a bad idea in terms of GPU memory allocation: we can easily collate this into a single buffer.
• modify_command_group should look to see if the command group has been transformed rather than graphically mutated so that we can simply modify by changing the transform matrix instead of re-tessellating.
• Still need to decide how to handle backdrop filters: 2 textures ping-ponging with various filter pipelines or 1 texture drawn once with pre-generated shader permutations (faster but WGPU doesn't have specialization constants which means generating strings at init-time).
• Provide two implementations: one for the main draw target and one for texture draw target. This should allow for retaining layers for performance, i.e. a scrollable area where scrolling = drawing texture at a different position vs redrawing everything inside. Would impede on renderer-agnosticity.
• Implement per-vertex AA by generating our own vertex normals and then using simple feathering in the shader. Not sure how feasible this is, seems more in the scope of Lyon.

Progress:

• [x] Displaying a regular GraphicsDisplayItem.
• [ ] Displaying TextDisplayItem (via wgpu_glyph).
• [ ] Clipping by a mesh (DisplayClip).
• [ ] Displaying BackdropFilter. Depends on clipping.
• [x] Save/Restore stack.
• [ ] SaveLayer.
• [ ] Transformations.

Enhancements:

• [ ] Look to see if a transformation matrix can be applied instead of re-tessellating in modify_command_group.
• [ ] Generate shader permutations for BackdropFilter rather than ping-ponging.

[jazzfool]

I'm going to close this, I took a stab at it, but maybe someone else with more experience can work on it. Reaching feature parity with Skia (backdrop filters, filters, text rendering, selective anti-aliasing, high quality GPU anti-aliasing) is way beyond my GPU programming knowledge.