[HTML5] Fix blurry, low FPS and crash when zooming.

[kan6868]

I have implemented this sample a few months ago, and it works well with various projects.

https://github.com/kan6868/s2d-html-template

• Fixing blurry images:
  I enlarged the game content by double and then reduced the canvas size by half.

// keep ratio
float scaleX = (w * 2) / (float)fWidth;
float scaleY = (h * 2) / (float)fHeight;

//reduced canvas size
emscripten_set_element_css_size("canvas", w / 2, h / 2);

• Fixing FPS flow issues:
  Based on this [Web] Frame Rate and UI fix #699

• Fixing crashes when zooming:
  I realized that at certain screen sizes, the width or height values of the content would return 0. So, I reassigned them.

if (w == 0 || h == 0) 
{
	w = jsContextGetWindowWidth();
	h = jsContextGetWindowHeight();
}