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.h

@@ -0,0 +1,6 @@
+#pragma once
+
+#include <stdint.h>
+
+void rgb_to_hsl(uint32_t rgb, int *hue, int *sat, int *lum);
+uint32_t hsl_to_rgb(int hue, int sat, int lum);