Fix rounding when un-premultiplying alpha in getImageData

[chiliec]

Fixes #1933

getImageData un-premultiplies semi-transparent pixels with (int)((float)r * ((float)255 / a)), which truncates. Because cairo's premultiplied channel is already rounded down, truncating again on the way back drops the value by one for most inputs: putImageData([252,252,252,252]) reads back as [251,251,251,252].

This switches to the integer round-half-up division that canvas_unpremultiply_data in src/PNG.h already uses ((c * 255 + a / 2) / a), so getImageData and PNG export un-premultiply identically. Fully transparent/opaque pixels keep the existing fast path.

Exhaustively checking every (value, alpha) pair with alpha in 1..254 (65,024 pixels): exact putImageData → getImageData round-trips go from 1,450 to 17,369, and no pixel gets worse; the remaining differences are the information lost by premultiplication itself.

• Have you updated CHANGELOG.md?

Validation (linux x64, zig 0.16.0, node 22.12):

zig build
npx mocha test/canvas.test.js -g 1933   # fails before the change: [251,251,251,252]
npm test                                # 306 passing, 6 pending