[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 #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.

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