sim: embedd background bmp and load from memory

Loading the file from disk introduces a slight dependency on the working
directory for the simulator. Prevent that by loading the bmp from
memory.

Use a python script to convert the backround bmp image to a C header
file. Include this header in `main.cpp` and load the bmp from memory.

img/convert_bmp_to_header.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+import argparse
+
+parser = argparse.ArgumentParser()
+
+parser.add_argument("bmp",
+    help="Path to bmp to convert to C header file")
+parser.add_argument("--var-name",
+   help="name of the variable to make the bmp data available",
+   default="IMAGE_DATA")
+parser.add_argument("-o", "--output",
+   help="Path where to create C header file",
+   required=True)
+args = parser.parse_args()
+
+with open(args.output, "w", encoding="utf-8") as f:
+    # conversion script based on:
+    # https://stackoverflow.com/questions/18422123/sdl-embed-image-inside-program-executable
+    f.write("static const unsigned char {:s}[] = {{{:s}}};".format(
+        args.var_name,
+        ",".join(str(b) for b in open(args.bmp, "rb").read())))