raphlinus
f32 -> u8 conversion should multiply by 256-ε instead of by 255
The Rust code has SIMD and non-SIMD versions of this conversion (255.0 * y) as u8 from an f32 in the range [0.0, 1.0] to a u8 in the range [0, 255].
Due to rounding errors, this sometimes means that a pixel well inside a glyph ends up with an 0xfe value instead of 0xff, since its floating point value is slightly less than 1.
In my Go port, I embiggened the constant from 255.0 to 255.99998, or 0x437fffff as a float32 bit pattern, which eliminated these central "0xfe"s. See the comment at the bottom of https://github.com/google/font-go/commit/7c31afdef0ec8d074834b29413aa097de0d51197
I think that the Rust code should do the same.
I'd suggest using centered quantization which is widely used in games, basically it can be implemented by using round instead of floor in the final step of calculation.
Though I don't think accuracy matters in the case of fonts (apart from avoiding triggering a different codepath due to 0xff turning into 0xfe), so your approach also seems OK.
