WebAssembly interface functions

We need to define the functions that will interact with our JavaScript. We also need to define some global variables that will be used by several of our classes. Here is the code from the new basic_particle.cpp file:

#include "game.hpp"
#include <emscripten/bind.h>
SDL_Window *window;
SDL_Renderer *renderer;
char* fileName;
Emitter* emitter = NULL;
Uint32 last_time = 0;
Uint32 current_time = 0;
Uint32 diff_time = 0;
float delta_time = 0.0f;
extern "C"
    EMSCRIPTEN_KEEPALIVE
    void add_emitter(char* file_name, int max_particles, float 
    min_angle, float max_angle, Uint32 particle_lifetime, float 
    acceleration, bool alpha_fade, float min_starting_velocity, float 
    kmax_starting_velocity, Uint32 emission_rate, float x_pos, float 
    y_pos, float radius) {
        if( emitter != NULL ) {
            delete emitter;
        }
  ...