add test file for tutorial

tests/hello_image_sdl.c

@@ -0,0 +1,43 @@
+#include <stdio.h>
+#include <SDL/SDL.h>
+#include <SDL/SDL_image.h>
+#include <assert.h>
+#include <emscripten.h>
+#include <unistd.h>
+
+int testImage(SDL_Surface* screen, const char* fileName) {
+  SDL_Surface *image = IMG_Load(fileName);
+  if (!image)
+  {
+     printf("IMG_Load: %s\n", IMG_GetError());
+     return 0;
+  }
+  assert(image->format->BitsPerPixel == 32);
+  assert(image->format->BytesPerPixel == 4);
+  assert(image->pitch == 4*image->w);
+  int result = image->w;
+
+  SDL_BlitSurface (image, NULL, screen, NULL);
+  SDL_FreeSurface (image);
+
+  return result;
+}
+
+int main() {
+  SDL_Init(SDL_INIT_VIDEO);
+  SDL_Surface *screen = SDL_SetVideoMode(600, 450, 32, SDL_SWSURFACE);
+
+  int result = 0;
+
+  result |= testImage(screen, "screenshot.jpg"); // absolute path
+  assert(result != 0);
+
+  SDL_Flip(screen);
+
+  printf("you should see an image.\n");
+
+  SDL_Quit();
+
+  return 0;
+}
+