toDataURL() now encodes the real pixel buffer as a valid PNG using
stored-block DEFLATE, CRC32, and Adler32. The 106-byte stub it replaced
was a static value that any canvas fingerprinter could trivially flag.
fill() rasterizes arc paths from this._path using a distance-check
circle fill, so arc-based canvas tests produce real pixel data.
_setPixel() handles globalCompositeOperation='multiply' so fpCollect's
colored-circle blend test composites correctly.
Canvas dimensions now read from getAttribute('width'/'height') so
explicitly sized canvases (e.g. 400x200) use their declared dimensions
instead of always defaulting to 300x150.
SGavrl
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