glide.generated event emitted even when image has already been generated

[hannahwoodward]

Describe the bug The 'glide.generated' event is called on every reference to a glide transformed image (not just on a 'fresh' generation of a transformed image).

To Reproduce Steps to reproduce the behavior:

1. Add an image to a page with an assets field.
2. Create an addon that listens for the glide.generated event
3. Call a glide transform on the image in the templates, e.g.

   {{ asset:some_image }}
   <img src="{{ glide:id width='500' height='500' }}">
   {{ /asset:some_image }}

Expected behavior I would expect glide.generated to only be emitted when a fresh glide image has been generated.

Environment details (please complete the following information):

• Statamic Version 2.11.15
• Fresh Install or Upgrade: Upgrade
• OS: macOS 10.14.3
• Browser: Any
• Web Server: Valet
• PHP Version: 7.3.4
• Addons installed: ColorPicker, Fetch, RemoteTransforms custom plugin (this is the plugin using the 'glide.generated' event)