Add sixel support

Originated from https://gist.github.com/CommandMaker/27ba97427424c06aff54d0a2a91a4ef1

sixel_hls.c

@@ -0,0 +1,115 @@
+// sixel.c (part of mintty)
+// this function is derived from a part of graphics.c
+// in Xterm pl#310 originally written by Ross Combs.
+//
+// Copyright 2013,2014 by Ross Combs
+//
+//                         All Rights Reserved
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+// IN NO EVENT SHALL THE ABOVE LISTED COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
+// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+// TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+// SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//
+// Except as contained in this notice, the name(s) of the above copyright
+// holders shall not be used in advertising or otherwise to promote the
+// sale, use or other dealings in this Software without prior written
+// authorization.
+
+#define SIXEL_RGB(r, g, b) ((r) + ((g) << 8) + ((b) << 16) + (255 << 24))
+
+int
+hls_to_rgb(int hue, int lum, int sat)
+{
+  double hs = (hue + 240) % 360;
+  double hv = hs / 360.0;
+  double lv = lum / 100.0;
+  double sv = sat / 100.0;
+  double c, x, m, c2;
+  double r1, g1, b1;
+  int r, g, b;
+  int hpi;
+
+  if (sat == 0) {
+    r = g = b = lum * 255 / 100;
+    return SIXEL_RGB(r, g, b);
+  }
+
+  if ((c2 = ((2.0 * lv) - 1.0)) < 0.0) {
+    c2 = -c2;
+  }
+  c = (1.0 - c2) * sv;
+  hpi = (int) (hv * 6.0);
+  x = (hpi & 1) ? c : 0.0;
+  m = lv - 0.5 * c;
+
+  switch (hpi) {
+  case 0:
+    r1 = c;
+    g1 = x;
+    b1 = 0.0;
+    break;
+  case 1:
+    r1 = x;
+    g1 = c;
+    b1 = 0.0;
+    break;
+  case 2:
+    r1 = 0.0;
+    g1 = c;
+    b1 = x;
+    break;
+  case 3:
+    r1 = 0.0;
+    g1 = x;
+    b1 = c;
+    break;
+  case 4:
+    r1 = x;
+    g1 = 0.0;
+    b1 = c;
+    break;
+  case 5:
+    r1 = c;
+    g1 = 0.0;
+    b1 = x;
+    break;
+  default:
+    return SIXEL_RGB(255, 255, 255);
+  }
+
+  r = (int) ((r1 + m) * 100.0 + 0.5);
+  g = (int) ((g1 + m) * 100.0 + 0.5);
+  b = (int) ((b1 + m) * 100.0 + 0.5);
+
+  if (r < 0) {
+    r = 0;
+  } else if (r > 100) {
+    r = 100;
+  }
+  if (g < 0) {
+    g = 0;
+  } else if (g > 100) {
+    g = 100;
+  }
+  if (b < 0) {
+    b = 0;
+  } else if (b > 100) {
+    b = 100;
+  }
+  return SIXEL_RGB(r * 255 / 100, g * 255 / 100, b * 255 / 100);
+}