Adding start button to initiate race

Makefile

exit_menu_gl.c

exit_menu_gl.h

glfw_utils.c

glfw_utils.h

image_gl.c

image_gl.h

images/start-selected.png

images/start.png

makefile_macos

renderer.c

renderer.h

shaders/texture_es.frag

shaders/texture_es.vert

start_menu_gl.c

start_menu_gl.h