Add support for responsive images

[gaborbsd]

Nowadays it is important to support responsive images to save on bandwidth and create fast websites. It is really easy to support this with just allowing the sizes and srcset attributes for the img tag generated.

I found glightbox really handy and easy to use, just I felt that this was missing and went on to add it myself. It was also easy to read and understand the code. Thank you so much for sharing this nice lib with the world and thanks in advance for your consideration on accepting this PR.

[gingerchew]

We have been discussing this a bit in #248, but I do think the check for sizes is a good idea.

Maybe it's worth it to merge the two like:

// pseudo-code
if (hasSrcset && hasSizes) {
  doSrcsetAndSizes()
}

Since one is pointless without the other.

[gaborbsd]

Thank you Frankie, I've modified the patch according to your suggestion. Btw, I forgot to mention that I also tested the change and it worked.

[tpeachman]

I wouldn't necessarily merge srcset and sizes. I mean why would you require it? From what I know, there are use cases where you wouldnt need the sizes attribute.
Regardless, please add the support asap :D

[iamrobert]

Thanks - the patch works well.

Here's my code implementation for a small, medium and large image sizes:

<a 
      href="images/large.jpg"  
      title="Responsive example" 
      data-title="Responsive example"
      data-description="Your browser will choose the optimal image for the resolution"
      class="glightbox"
      data-sizes="(max-width: 600px) 480px, (max-width: 1024px), (min-width: 1024px)"
      data-srcset="images/small.jpg 480w, images/medium.jpg 1023w, images/bb_files/large.jpg 1024w">
      GLIGHTBOX
</a>

[iamrobert]

@Frankie-tech Should this branch be in the /dist/ folder?

[gaborbsd]

Thanks for merging this!

[gingerchew]

@qpido You are correct, you could also use it alone for just pixel density.

@iamrobert You are also right, the build script should have been run.

I'll make a new PR later today to fix both of those.