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().

meson.build

@@ -111,6 +111,7 @@ executable(
   'extract.c', 'extract.h',
   'fdm.c', 'fdm.h',
   'grid.c', 'grid.h',
+  'hsl.c', 'hsl.h',
   'input.c', 'input.h',
   'log.c', 'log.h',
   'macros.h',
@@ -125,7 +126,6 @@ executable(
   'server.c', 'server.h',
   'shm.c', 'shm.h',
   'sixel.c', 'sixel.h',
-  'sixel-hls.c', 'sixel-hls.h',
   'slave.c', 'slave.h',
   'spawn.c', 'spawn.h',
   'terminal.c', 'terminal.h',