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

include/img/img-xpm.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_IMG_XPM_H
+#define LABWC_IMG_XPM_H
+
+struct lab_data_buffer;
+
+void img_xpm_load(const char *filename, struct lab_data_buffer **buffer);
+
+#endif /* LABWC_IMG_XPM_H */