Incremental rendering

[hecrj]

This PR implements basic incremental rendering for the upcoming software renderer powered by tiny-skia.

Basically, we compare the graphical primitives of the last drawn frame with the current primitives and produce a list of rectangles representing the damaged regions of the frame. Then, this list is reduced by grouping rectangles in a certain distance threshold together. Finally, when drawing, we simply skip any primitive that does not intersect with any damaged region.

The following video shows the tour example but old primitives fade with time and the red rectangles are the grouped damage regions:

2023-04-05.06-31-02.mp4

The diffing logic of graphical primitives is generic enough and could be eventually reused in iced_wgpu.

[i509VCB]

If a single Backend were able to be used multiple times (for multiple windows if that was intended) you would end up with a situation where the second window is fully redrawn because the primitives may be completely different on that other window.

[hecrj]

Certainly! This is a hint that damage tracking should actually happen at the compositor level. Fixed in af0303f.

[ids1024]

rust-windowing/softbuffer#99 now has APIs for buffer age and damage (where supported), so incremental rendering can be done with no-copy presentation. On some platforms, anyway.

On Wayland this uses double-buffering, so age can be None, 1, or 2, and you need to be able to update the parts of the surface that have changed in the past two frames. On MacOS currently there's a new buffer time so age is always None. (rust-windowing/softbuffer#95 or rust-windowing/softbuffer#96 would fix that). Other platforms currently use a single buffer so age is None or 1.

For Wgpu, it looks like there's no API for damage or anything that exposes the buffer age, or a way to update only part of it? gfx-rs/wgpu#2869 discusses damage, I don't see any mention of a feature like EGL_EXT_buffer_age.

[hecrj]

@ids1024 Cool! It looks like all of those features are still being worked on. Let's wait a bit until everything settles!

[ids1024]

rust-windowing/softbuffer#99 should work, though I've left it as a draft since it's still entirely untested. That presumably should help with incremental rendering performance on Windows and Linux.