hsl: add our own implementations of rgb-to-hsl and hsl-to-rgb

* New function: rgb_to_hsl()
* New function: hsl_to_rgb()
* Replace XTerm’s hls_to_rgb() with our own, hsl_to_rgb().
* Ensure hue/lum/sat values are within range before calling
  hsl_to_rgb()

Note that sixels’ use the following primary hues:

*  blue:  0°
*  red:   120
*  green: 240°

While “standard” HSL uses:

*  red:   0°
*  green: 120°
*  blue:  240°

Thus, we need to adjust the sixel’s hue value before calling
hsl_to_rgb().

hsl.c

@@ -0,0 +1,89 @@
+#include "hsl.h"
+
+#include <math.h>
+
+#include "util.h"
+
+void
+rgb_to_hsl(uint32_t rgb, int *hue, int *sat, int *lum)
+{
+    double r = (double)((rgb >> 16) & 0xff) / 255.;
+    double g = (double)((rgb >> 8) & 0xff) / 255.;
+    double b = (double)((rgb >> 0) & 0xff) / 255.;
+
+    double x_max = max(max(r, g), b);
+    double x_min = min(min(r, g), b);
+    double V = x_max;
+
+    double C = x_max - x_min;
+    double L = (x_max + x_min) / 2.;
+
+    *lum = 100 * L;
+
+    if (C == 0.0)
+        *hue = 0;
+    else if (V == r)
+        *hue = 60. * (0. + (g - b) / C);
+    else if (V == g)
+        *hue = 60. * (2. + (b - r) / C);
+    else if (V == b)
+        *hue = 60. * (4. + (r - g) / C);
+    if (*hue < 0)
+        *hue += 360;
+
+    double S = C == 0.0
+        ? 0
+        : C / (1. - fabs(2. * L - 1.));
+    *sat = 100 * S;
+}
+
+uint32_t
+hsl_to_rgb(int hue, int sat, int lum)
+{
+    double L = lum / 100.0;
+    double S = sat / 100.0;
+    double C = (1. - fabs(2. * L - 1.)) * S;
+
+    double X = C * (1. - fabs(fmod((double)hue / 60., 2.) - 1.));
+    double m = L - C / 2.;
+
+    double r, g, b;
+    if (hue >= 0 && hue <= 60) {
+        r = C;
+        g = X;
+        b = 0.;
+    } else if (hue >= 60 && hue <= 120) {
+        r = X;
+        g = C;
+        b = 0.;
+    } else if (hue >= 120 && hue <= 180) {
+        r = 0.;
+        g = C;
+        b = X;
+    } else if (hue >= 180 && hue <= 240) {
+        r = 0.;
+        g = X;
+        b = C;
+    } else if (hue >= 240 && hue <= 300) {
+        r = X;
+        g = 0.;
+        b = C;
+    } else if (hue >= 300 && hue <= 360) {
+        r = C;
+        g = 0.;
+        b = X;
+    } else {
+        r = 0.;
+        g = 0.;
+        b = 0.;
+    }
+
+    r += m;
+    g += m;
+    b += m;
+
+    return (
+        (int)round(r * 255.) << 16 |
+        (int)round(g * 255.) << 8 |
+        (int)round(b * 255.) << 0);
+}