Avoid trying to shrink indexed-color PNG files (BL-16424)

[StephenMcConnel]

These are the primary source of files that don't shrink when the image dimensions are reduced. The code now tries to do this twice when pasting an image, causing a lot of slowdown when shrinking fails on nontrivial images.

Devin review

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This change is 

[greptile-apps]

Greptile Summary

Adds an early-exit heuristic in TryResizeImageWithGraphicsMagick to skip GraphicsMagick processing for indexed-color PNG files, which lose their compact format on conversion to RGBA and therefore frequently end up larger after a resize operation.

• IsIndexedColorPngFile reads the first 26 bytes of the PNG file to validate the signature and inspect the IHDR color-type byte (offset 25 = 3 for indexed color); it is called as a fallback when the in-memory PixelFormat.Indexed flag is absent (e.g., clipboard paste).
• The guard bails out early (returning null) when the estimated uncompressed size of the converted RGBA output would not be smaller than the source, while still allowing GraphicsMagick through when the resolution reduction is large enough to justify the format change.

Important Files Changed

Filename	Overview
src/BloomExe/ImageProcessing/ImageUtils.cs	Adds IsIndexedColorPngFile helper and indexed-PNG early-exit guard in TryResizeImageWithGraphicsMagick; logic is sound but has a minor robustness gap in the header read and an imprecise bit-depth assumption in the area heuristic comment.

_{Reviews (1): Last reviewed commit: "Avoid trying to shrink indexed-color PNG..." | Re-trigger Greptile}

[greptile-apps]

Stream.Read may return fewer bytes than requested even when the stream has enough data, so header[25] could end up zero (causing a false negative) if the read is split across multiple system calls. Capturing the return value and looping — or using ReadExactly — ensures the full 26-byte header is always populated before the color-type check.

Suggested change

	using (var fs = RobustIO.GetFileStream(filePath, FileMode.Open, FileAccess.Read))
	{
	if (fs.Length < 26)
	return false;
	fs.Read(header, 0, 26);
	}
	using (var fs = RobustIO.GetFileStream(filePath, FileMode.Open, FileAccess.Read))
	{
	if (fs.Length < 26)
	return false;
	int offset = 0;
	while (offset < 26)
	{
	int read = fs.Read(header, offset, 26 - offset);
	if (read == 0)
	return false; // unexpected EOF
	offset += read;
	}
	}

[greptile-apps]

Area heuristic assumes 8 bpp for all indexed PNGs

Indexed PNGs can legally use 1, 2, or 4 bits per pixel (PNG spec §11.2.2). The comment "1 byte per pixel for indexed PNGs" is only accurate for 8 bpp; for lower bit-depth images currentArea overestimates the source footprint by up to 8×, so the guard is more conservative than intended — it will refuse to convert cases where conversion might actually shrink the file. The downstream file-size check still prevents any wrongly-approved conversion from landing, so behavior is always correct, but some beneficial resizes may be skipped for small low-bpp palettised images.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!