add new "MEDIA_RESIZE" event

[takuy]

Not sure if this is the best approach but would close #3032.

This adds a single "MEDIA_RESIZE" event called in fetch.php for when an image is requested by a client.

[phy25]

Generally LGTM but I am not sure if we want an event for this.

[takuy]

Generally LGTM but I am not sure if we want an event for this.

There's a few things I personally would want to change/add configuration options for in terms of how things are resized (or skip it entirely depending on context) that would be doable as plugins if this event is added:

• disable server-side resizing for a specific file extension (which I also have written here, not sure if I want to submit that as a separate PR yet (takuy@dfa769b)
• disable server-side resizing for internal images if it has nocache set
• write a completely different resize algorithm using another tool (ie, not use GD or ImageMagick)
• resize specific file types using different methods (ie, not use GD or ImageMagick)

[splitbrain]

@takuy can you add the necessary documentation in the wiki please?

[takuy]

@takuy can you add the necessary documentation in the wiki please?

Thanks @splitbrain, I've created devel:event:media_resize for this event. Let me know if there's any additional document required.