Fix distance grid overflow for glyphs clipped by the canvas edge

[cherenkov]

draw() clips an overflowing glyph to size - buffer, so its padded box reaches size + buffer — one buffer wider than the canvas. The grids and the f/v/z scratch arrays are sized from size, so edt1d runs past their end and the clipped edges come out zeroed.

This is a regression of #29, reintroduced in e83b8c3 when gridSize was folded into size. The two aren't interchangeable: the grid is always size + buffer, because the clip bound is itself derived from size — growing the canvas grows the largest glyph by the same amount.

Repro — {fontSize: 20, buffer: 2} (28px canvas), a glyph filling it, padded box 30×30:

before                        after
128 159 223 255   0   0   0   128 159 223 255 255 255 255
  0   0   0   0   0   0   0   128 159 223 223 223 223 223
  0   0   0   0   0   0   0   120 146 159 159 159 159 159

Only glyphs whose bounding box exceeds size - 2 * buffer are affected, so which ones depends on the renderer and the font — it comes down to how large a bounding box the font reports relative to fontSize. In Chrome/macOS sans-serif at the defaults it is Arabic presentation forms: 2.0% of U+FB50–FDFF at fontSize: 24 and 4.9% at 48, up to 3.5% of a glyph's bytes. Latin, Greek, CJK, kana, Hangul and emoji all come out clean there. Under node-canvas the same code corrupts ~55% of the emoji block instead, because emoji measure 1.33em there against Chrome's 1.13em.

Either way it matches the note on #29 that this "happens to be correct for almost any glyph (thus we didn't notice the problem)".

Fix — name the clip bound maxGlyphDim and derive the grid dimension from it, so the two can't drift apart again. Output is byte-identical for every glyph that wasn't already corrupted (565 renders across 5 option sets). Costs ~3.6 KB per instance at the defaults (the two grids go from 36² to 39²).

Test — rasterizes a canvas-filling square through stubbed measureText/fillText, so it needs no particular font. Asserts the padded glyph fits every array and that the SDF stays symmetric. Fails on main.

[ox-security]

OX Security reviewed this pull request — nothing to fix.

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Branch fix/clipped-glyph-grid-overflow → main

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