perf(desktop-gpui): encode screenshot save and copy with the parallel PNG writer

[kyzer1023]

Summary

encode_png_rgba (used by screenshot save, copy, transparent share, and the screenshot editor's PNG export) went through the image crate's single-threaded PNG encoder. It now calls the in-tree png::encode (zlib level 6, parallel strips), the same path write_png_file already uses for default compression with adaptive filtering. The CompressionType::Fast capture path is unchanged.

One commit, one file: apps/desktop-gpui/kernels/src/codec.rs. Based on main at 85219015c.

Correctness

Decoded pixels match the previous encoder. cargo +stable test -p cap-gpui-kernels passes (41 tests), including a 16×16 frame of every alpha value (the pattern unannotated_composite_and_copy_preserve_all_alpha_values encodes) and the existing 3024×1964 strip round trip. PNG size stays within about 0.1%.

The full desktop-gpui binary was not built here because gpui is patched to a local zed-cap checkout.

Benchmarks

kernels/benches/png_encode.rs, stable rustc 1.99.1, release, 4 vCPU. Bench median of 10 samples; the bench asserts decoded pixels equal the source.

UI-like RGBA (toolbar, solid window, translucent edge), median of five runs:

Size	Before	After	Speedup
3024×1964	41.6 ms	24.0 ms	1.73×
3840×2160	57.7 ms	34.8 ms	1.66×

Detailed RGBA (screen_like), two runs:

Size	Before	After	Speedup
3024×1964	223.3 / 243.7 ms	68.7 / 77.4 ms	3.25× / 3.15×
3840×2160	305.4 / 305.0 ms	94.6 / 94.6 ms	3.23× / 3.22×

Flat frames have less deflate work, so the gain is smaller there. More cores should widen the gap (the writer uses up to 8 strips).

Overlap

Does not touch filter_rows / adaptive filter code (#2674), rgba_to_rgb / encode_jpeg_rgba_as_rgb (#2672), or capture write_png (#2703).

Confidence Score: 5/5

This PR appears safe to merge; no actionable issues were found.

Summary

encode_png_rgba now uses the existing parallel PNG writer for screenshot save, copy, transparent share, and PNG export.

• Screenshot PNG exports use the parallel encoder.

_{Reviews (1) · Last reviewed commit: "perf(desktop-gpui): encode screenshot PN..." · Reviewed by Greptile}

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