icon-loader: add a new loader for XPM icons, used by many legacy X11 apps

Adapted from gdk-pixbuf, see the original at:

  https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/blob/master/gdk-pixbuf/io-xpm.c

rgb.txt is from X.org, see:

  https://gitlab.freedesktop.org/xorg/app/rgb/-/blob/master/rgb.txt

Differences from the gdk-pixbuf version:

- GdkPixbuf replaced with struct lab_data_buffer
- Progressive and in-memory loading code removed
- Two functions that had separate BSD copyright rewritten
- Stores colors as ARGB32 earlier in the decoding process for efficiency
- Limited to 1024x1024 px and 1024 colors to prevent abuse
- Uses struct buf to build strings, in place of manual g_new/g_realloc
- Uses xzalloc/xznew_n for other memory allocations
- Uses g_strlcpy in place of banned strcpy/strncpy/strncat
- Uses standard C types (int, bool, etc.) in place of the GLib ones
- Follows labwc coding style (whitespace, braces, letter case, etc.)
- Et cetera ...

v2: add Perl fixes from @domo141

src/img/gen-color-table.pl

@@ -0,0 +1,80 @@
+#!/usr/bin/perl -w
+
+# Generates xpm-color-table.h from X11's rgb.txt
+# Adapted from gdk-pixbuf (LGPL-2.0-or-later)
+
+if (@ARGV != 1) {
+    die "Usage: gen-color-table.pl rgb.txt > xpm-color-table.h\n";
+}
+
+open IN, '<', $ARGV[0] or die "Cannot open $ARGV[0]: $!\n";
+
+@colors = ();
+while (<IN>) {
+    next if /^!/;
+    if (!/^\s*([0-9]+)\s+([0-9]+)\s+([0-9]+)\s+(.*\S)\s*$/) {
+	die "Cannot parse line $_";
+    }
+
+    push @colors, [$1, $2, $3, $4];
+}
+
+close IN or die "close IN failed: $!\n";
+
+@colors = sort { lc($a->[3]) cmp lc($b->[3]) } @colors;
+
+$offset = 0;
+
+$date = gmtime;
+
+print <<EOT;
+/* SPDX-License-Identifier: LGPL-2.0-or-later */
+/* xpm-color-table.h: Generated by gen-color-table.pl from rgb.txt
+ *
+ *  Date: $date
+ *
+ * Do not edit.
+ */
+static const char color_names[] =
+EOT
+
+for $color (@colors) {
+    $name = $color->[3];
+
+    if ($offset != 0) {
+	print qq(\n);
+    }
+    print qq(	"$name\\0");
+
+    $color->[4] = $offset;
+    $offset += length($name) + 1;
+}
+
+print ";\n\n";
+
+print <<EOT;
+struct xcolor_entry {
+	uint16_t name_offset;
+	uint8_t red;
+	uint8_t green;
+	uint8_t blue;
+};
+
+static const struct xcolor_entry xcolors[] = {
+EOT
+
+$i = 0;
+for $color (@colors) {
+    $red = $color->[0];
+    $green = $color->[1];
+    $blue = $color->[2];
+    $offset = $color->[4];
+
+    if ($i != 0) {
+	print ",\n";
+    }
+    print "	{ $offset, $red, $green, $blue }";
+    $i++;
+}
+
+print "\n};\n";