fix: GetGlyph falls back to index 0 when rune is not found

[rdon-key]

This pull request fixes an issue where GetGlyph could return the glyph of the next larger rune if the requested rune was not found in the font.
As a result, undefined characters could be rendered as completely unrelated glyphs (see #58).

Changes

• Updated GetGlyph in both const1bit and const2bit packages:
  • Added an equality check (==) during the binary search.
  • Introduced a found flag to ensure that if no match is found, the function always falls back to index 0.
• Updated function comments to explicitly state that undefined runes return the glyph at index 0.

Verification

• Verified with const2bit fonts (WebAssembly rendering).
• Could not test with const1bit fonts locally, but the implementation is identical.

tinyfont.WriteLine(target, &testfont, 10, 50, "①②③④⑤", color.RGBA{0, 0, 0, 255})

Impact

• Undefined runes will no longer render as unrelated valid glyphs.
• Instead, they consistently fall back to glyph 0 (commonly a space or placeholder).

Fixes #58