Remove (and fix) clippy locally disabled features

[mpizenberg]

They are easily found by searching for "clippy". Mostly, the disabled features are:

• excessive_precision: typically, f32 floats with too many figures. Those should be removed when the corresponding code becomes generic over f32 and f64.
• many_single_char_names: happens in most "bloc" functions where a 1-letter name is given for each sub pixel to make equations readable. It should probably be smarter to use at least 2 letters though. Something like tl, bl, tr, br for top left, bottom left, etc.
• cast warnings.