move stb_image/stb_image_write.cpp into a cpp file instead of random files with the magic 'STB_IMAGE_WRITE_IMPLEMENTATION' define

move setup.py back to eglRenderer extension, use pkgutil.get_loader('eglRenderer').get_filename()
disable dlmopen by default, unless B3_USE_DLMOPEN is defined.

examples/OpenGLWindow/opengl_fontstashcallbacks.cpp

@@ -9,7 +9,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#define STB_IMAGE_WRITE_IMPLEMENTATION
 #include "stb_image/stb_image_write.h"