Cache-busting hash can remain unchanged despite content changing

Problem/Motivation

Even when content of an icon sprite sheet changes, the cache-busting query hash can remain the same. This is due to generation of the hash using only some plugin data but not the actual file data.

Steps to reproduce

1. Create a sprite sheet with some icons.
2. Render an icon, taking note of the cache-busting hash
3. Change content of the sprite sheet, without changing any viewboxes (just the path content)
4. Clear the Drupal cache, see that the hash remains the same

Proposed resolution

Use the file content to generate the hash.

Remaining tasks

Make the patch.