examples : use colorblind friendly TTY color scheme (#2360)

This change updates the -pc flag, so that a new xterm256 color scheme is
used. This color scheme is believed to be better for three reasons:

1. It should be friendlier to the colorblind. The scheme was designed by
Paul Tol (see: https://personal.sron.nl/~pault/). TensorBoard uses it
since 2017, so it's already popular in the machine learning community

2. It should appear to be the same colors as before to people who aren't
i.e. it's still a red-green spectrum like before but lightly modified

3. It is readable in both white and black background terminals. The neon
colors before were probably a bit too intense for white backgrounds.

examples/common.h

@@ -9,6 +9,7 @@
 #include <thread>
 #include <ctime>
 #include <fstream>
+#include <sstream>
 
 #define COMMON_SAMPLE_RATE 16000
 
@@ -286,12 +287,43 @@ void sam_print_usage(int argc, char ** argv, const sam_params & params);
 // Terminal utils
 //
 
-
-// Terminal color map. 10 colors grouped in ranges [0.0, 0.1, ..., 0.9]
-// Lowest is red, middle is yellow, highest is green.
+#define SQR(X)    ((X) * (X))
+#define UNCUBE(x) x < 48 ? 0 : x < 115 ? 1 : (x - 35) / 40
+
+/**
+ * Quantizes 24-bit RGB to xterm256 code range [16,256).
+ */
+static int rgb2xterm256(int r, int g, int b) {
+    unsigned char cube[] = {0, 0137, 0207, 0257, 0327, 0377};
+    int av, ir, ig, ib, il, qr, qg, qb, ql;
+    av = r * .299 + g * .587 + b * .114 + .5;
+    ql = (il = av > 238 ? 23 : (av - 3) / 10) * 10 + 8;
+    qr = cube[(ir = UNCUBE(r))];
+    qg = cube[(ig = UNCUBE(g))];
+    qb = cube[(ib = UNCUBE(b))];
+    if (SQR(qr - r) + SQR(qg - g) + SQR(qb - b) <=
+        SQR(ql - r) + SQR(ql - g) + SQR(ql - b))
+        return ir * 36 + ig * 6 + ib + 020;
+    return il + 0350;
+}
+
+static std::string set_xterm256_foreground(int r, int g, int b) {
+    int x = rgb2xterm256(r, g, b);
+    std::ostringstream oss;
+    oss << "\033[38;5;" << x << "m";
+    return oss.str();
+}
+
+// Lowest is red, middle is yellow, highest is green. Color scheme from
+// Paul Tol; it is colorblind friendly https://personal.sron.nl/~pault/
 const std::vector<std::string> k_colors = {
-    "\033[38;5;196m", "\033[38;5;202m", "\033[38;5;208m", "\033[38;5;214m", "\033[38;5;220m",
-    "\033[38;5;226m", "\033[38;5;190m", "\033[38;5;154m", "\033[38;5;118m", "\033[38;5;82m",
+    set_xterm256_foreground(220,   5,  12),
+    set_xterm256_foreground(232,  96,  28),
+    set_xterm256_foreground(241, 147,  45),
+    set_xterm256_foreground(246, 193,  65),
+    set_xterm256_foreground(247, 240,  86),
+    set_xterm256_foreground(144, 201, 135),
+    set_xterm256_foreground( 78, 178, 101),
 };
 
 //