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/CMakeLists.txt

@@ -0,0 +1,26 @@
+message(STATUS "folder img: converting background.bmp to C file to include in binary")
+
+if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12)
+   # FindPython3 module introduces with CMake 3.12
+   # https://cmake.org/cmake/help/latest/module/FindPython3.html
+   find_package(Python3 REQUIRED)
+else()
+   set(Python3_EXECUTABLE "python")
+endif()
+
+set(bmp_file ${CMAKE_CURRENT_SOURCE_DIR}/sim_background.bmp)
+set(out_file ${CMAKE_CURRENT_BINARY_DIR}/sim_background.h)
+# call python script to convert image to c file
+add_custom_command(
+   OUTPUT ${out_file}
+   COMMAND "${Python3_EXECUTABLE}" ${CMAKE_CURRENT_SOURCE_DIR}/convert_bmp_to_header.py
+   ${bmp_file}
+   --var-name "SIM_BACKGROUND"
+   --output ${out_file}
+   DEPENDS ${bmp_file}
+   WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+)
+# add target that can be added as dependency to infinisim target to trigger creation of C-file
+add_custom_target(infinisim_img_background
+   DEPENDS ${out_file}
+)