QColor: rewrite toHsv() to avoid float comparisonsHEADdev

The input data is integral. So keep using integral types for as long
as possible and only convert to float when performing a calculation
that requires fractional results.

This avoids the need for qFuzzyCompare(), which has a pre-condition
that neither of its argument must be zero, which an RGB value cannot
guarantee.

Here is why this works:

The old code immediately scaled the r, g, b values from 0...USHRT_MAX
to 0..1 float values. So, r, g, b, min, max, and delta variables were
rescaled. The new code doesn't scale said variables. So fractions
(like `delta / max` or `(g - b) / delta` remain mathematically the
same (and numerically, too, if the division is done in FP), and so
both `hue` as well as `saturation` don't change value.

There's another copy of this code that needs the same treatment (for
HSL), but they are sufficiently different from each other to take that
in a follow-up patch.

Also:

- Move the assertion into the third leg of the if-else chain. Now that
  we use precise integer arithmetic, it cannot happen that none of
  r, g, b are equal to max({r,g,b}), so if it wasn't equal to r or g,
  it must be equal to b. This allows us to leave `hue` partially-formed,
  so the compiler can warn about it being used uninitialized.
- clean up overparenthesization, whitespace and make the individual
  legs of the if-else chain look more similar by using an explicit 0
  literal in the first

Drive-by changes:

- use std::minmax(initializer_list) instead of our own Q_MIN_3/Q_MAX_3
  macros (the macros can't be removed, yet, as they're still used in
  toHsl()).
- reduce the scope of `delta`, after replacing the check for 0 with an
  integral one

As a consequence of using less FP arithmetic, tst_bench_qcolor's
toHsv() runtime goes down from 230ms (best-of-10) to 160 (ditto), a
whopping 40% reduction.

Amends the start of the public history.

Pick-to: 6.11 6.10 6.8 6.5
Task-number: QTBUG-142020
Change-Id: I370e8a214e48479b0c6fd0e48eb8f43c66920103
Reviewed-by: Edward Welbourne <[email protected]>

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