Utilize HTML loading="lazy" attribute instead of current lazy loading implementation

[thomasdn]

Galleria seems to implement its own JS version of lazy loading images.

For projects that utilize Galleria to show thumbnails, this crashes Safari pages on iOS devices ( see saimn/sigal#497)

I believe the custom JS implementation of lazy loading could be omitted entirely and replaced by built in HTML features for lazy loading, namely:

<img src="image.jpg" alt="foo" loading="lazy" />

I suppose this HTML5 feature was not available when Galleria implemented the JS lazy loading. Now that it is available, it would probably be better to use standard functionality. It would likely also perform better - although, I have not been able to test this.

Would it be feasible to make this change?