Fix panic in Bitmap::decode on zero-width/height EBDT glyphs

[youdie006]

Summary

Fixes #139.

Bitmap::decode panics with chunk size must be non-zero on bitmap glyphs whose width (or height) is 0. Such glyphs are valid font data -- blank glyphs in embedded bitmap strikes (EBLC/EBDT) of CJK fonts such as SimSun (simsun.ttc). Any caller using Source::Bitmap(..) / Scaler::scale_bitmap* on such a font hits the panic while rendering ordinary text.

Root cause

For w == 0, the row stride ((w * bits) + 7) / 8 evaluates to 0, and slice::chunks(0) panics by contract. This happens in all three BitmapFormat::Alpha(1|2|4) branches (src/strike.rs). decoded_size() is also 0, so the earlier target.len() < size check does not filter these out. FreeType returns an empty bitmap for these glyphs rather than failing, so native apps render such fonts without issues.

Fix

Guard decode for zero-sized bitmaps and return an empty (successful) decode before reaching the row loop:

if w == 0 || h == 0 {
    return true;
}

Semantics: true (empty decode) vs false (fallback)

The issue suggested two options. This PR returns true (decode succeeds with an empty image), for two reasons:

1. FreeType returns an empty bitmap, not a failure, for these glyphs.
2. This crate's own bitmap::resize already does exactly if target_width == 0 || target_height == 0 { return true; } (src/scale/bitmap/mod.rs), treating zero dimensions as a successful empty result -- so decode returning true matches the existing convention.

The alternative is return false, which makes the caller fall back to the next Source (e.g. Outline). Happy to switch to false if you'd prefer blank bitmap glyphs to fall through to outline rendering -- let me know which semantics you want.

Test

Added an in-crate regression test (decode_zero_width_bitmap_does_not_panic) covering Alpha(1|2|4) zero-width bitmaps plus a zero-height case. Verified red/green:

• Without the fix: panics at src/strike.rs:439: chunk size must be non-zero.
• With the fix: passes; full cargo test green; cargo fmt --check clean; no new cargo clippy warnings.

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This contribution was prepared with AI assistance and reviewed by me before submission.