psd: conversion speed improvements (kf5)

- Improved performance converting CMYK files by \~10% by replacing divisions with multiplications.
- Improved performance converting LAB files by \~50% by replacing std::pow with fastPow (approximated pow function).

src/imageformats/fastmath_p.h

@@ -0,0 +1,35 @@
+/*
+    Approximated math functions used into conversions.
+
+    SPDX-FileCopyrightText: Edward Kmett
+    SPDX-FileCopyrightText: 2023 Mirco Miranda <mircomir@outlook.com>
+
+    SPDX-License-Identifier: BSD-3-Clause
+*/
+#ifndef FASTMATH_P_H
+#define FASTMATH_P_H
+
+#include <QtGlobal>
+
+/*!
+ * \brief fastPow
+ * Based on Edward Kmett code released into the public domain.
+ * See also: https://github.com/ekmett/approximate
+ */
+inline double fastPow(double x, double y)
+{
+    union {
+        double d;
+        qint32 i[2];
+    } u = {x};
+#if Q_BYTE_ORDER == Q_LITTLE_ENDIAN
+    u.i[1] = qint32(y * (u.i[1] - 1072632447) + 1072632447);
+    u.i[0] = 0;
+#else // never tested
+    u.i[0] = qint32(y * (u.i[0] - 1072632447) + 1072632447);
+    u.i[1] = 0;
+#endif
+    return u.d;
+}
+
+#endif // FASTMATH_P_H