This is another attempt to solve the same problem as
https://github.com/zed-industries/zed/pull/29718, while avoiding the
regression on Intel GPUs.
### Background
Currently, on main, all paths are first rendered to an intermediate
"atlas" texture, similar to what we use for rendering glyphs, but with
multi-sample antialiasing enabled. They are then drawn into our actual
frame buffer in a separate pass, via the "path sprite" shaders.
Notably, the intermediate texture acts as an "atlas" - the paths are
laid out in a non-overlapping way, so that each path could be copied to
an arbitrary position in the final scene. This non-overlapping approach
makes a lot sense for Glyphs (which are frequently re-used in multiple
places within a frame, and even across frames), but paths do not have
these properties.
* we clear the atlas every frame
* we rasterize each path separately. there is no deduping.
The problem with our current approach is that the path atlas textures
can end up using lots of VRAM if the scene contains many paths. This is
more of a problem in other apps that use GPUI than it is in Zed, but I
do think it's an issue for Zed as well. On Windows, I have hit some
crashes related to GPU memory.
In https://github.com/zed-industries/zed/pull/29718, @sunli829
simplified path rendering to just draw directly to the frame buffer, and
enabled msaa for the whole frame buffer. But apparently this doesn't
work well on Intel GPUs because MSAA is slow on those GPUs. So we
reverted that PR.
### Solution
With this PR, we rasterize paths to an intermediate texture with MSAA.
But rather than treating this intermediate texture like an *atlas*
(growing it in order to allocate non-overlapping rectangles for every
path), we simply use a single fixed-size, color texture that is the same
size as thew viewport. In this texture, we rasterize the paths in their
final screen position, allowing them to overlap. Then we simply blit
them from the resolved texture to the frame buffer.
### To do
* [x] Implement for Metal
* [x] Implement for Blade
* [x] Fix content masking for paths
* [x] Fix rendering of partially transparent paths
* [x] Verify that this performs well on Intel GPUs (help @notpeter 🙏 )
* [ ] Profile and optimize
Release Notes:
- N/A
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Co-authored-by: Junkui Zhang <364772080@qq.com>
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