Update test_sdl_canvas

When I run this test locally in firefox I was getting 2785 as the
sum value.  I believe this is because the test includes SDL1 font
rendering which can depend on the system fonts you have installed.

test/browser/test_sdl_canvas.c

@@ -50,34 +50,33 @@ int main(int argc, char **argv) {
   SDL_Rect rect = { 200, 200, 175, 125 };
   SDL_FillRect(screen, &rect, SDL_MapRGBA(screen->format, 0x22, 0x22, 0xff, 0xff));
 
-  SDL_Flip(screen); 
+  SDL_Flip(screen);
 
   SDL_LockSurface(screen);
 
   int width, height;
   emscripten_get_canvas_element_size("#canvas", &width, &height);
-
-  if (width != 600 && height != 450)
-  {
-    printf("error: wrong width/height\n");
-    abort();
-  }
+  assert(width == 600);
+  assert(height == 450);
 
   int sum = 0;
+  int row_size = screen->w * 4;
+  // Sum the red components of each pixel on the diagonal.
   for (int i = 0; i < screen->h; i++) {
-    sum += *((char*)screen->pixels + i*screen->w*4 + i*4 + 0);
+    sum += *((char*)screen->pixels + i*row_size + i*4 + 0);
   }
   printf("Sum: %d\n", sum);
 
   printf("you should see two lines of text in different colors and a blue rectangle\n");
 
   SDL_UnlockSurface(screen);
-  
+
   SDL_Quit();
 
   printf("done.\n");
 
-  assert(sum > 3000 && sum < 5000); // varies a little on different browsers, font differences?
+  // varies a little on different browsers, font differences?
+  assert(sum > 2000 && sum < 5000);
 
   return 0;
 }

test/test_browser.py

@@ -831,13 +831,13 @@ def test_sdl_stb_image_cleanup(self):
     shutil.copyfile(test_file('screenshot.jpg'), 'screenshot.not')
     self.btest_exit('test_sdl_stb_image_cleanup.c', args=['-sSTB_IMAGE', '--preload-file', 'screenshot.not', '-lSDL', '-lGL', '--memoryprofiler'])
 
-  def test_sdl_canvas(self):
-    self.btest_exit('test_sdl_canvas.c', args=['-sLEGACY_GL_EMULATION', '-lSDL', '-lGL'])
-    # some extra coverage
-    self.clear()
-    self.btest_exit('test_sdl_canvas.c', args=['-sLEGACY_GL_EMULATION', '-O0', '-sSAFE_HEAP', '-lSDL', '-lGL'])
-    self.clear()
-    self.btest_exit('test_sdl_canvas.c', args=['-sLEGACY_GL_EMULATION', '-O2', '-sSAFE_HEAP', '-lSDL', '-lGL'])
+  @parameterized({
+    '': ([],),
+    '_safe_heap': (['-sSAFE_HEAP'],),
+    '_safe_heap_O2': (['-sSAFE_HEAP', '-O2'],),
+  })
+  def test_sdl_canvas(self, args):
+    self.btest_exit('test_sdl_canvas.c', args=['-sLEGACY_GL_EMULATION', '-lSDL', '-lGL'] + args)
 
   def post_manual_reftest(self):
     assert os.path.exists('reftest.js')