Don't allow images to drop while they're in-flight

[ryanisaacg]

This was causing images to be destructed before they hit the shader.

Resolves #601

Checks

• [x] I have updated CHANGES.md, with [BREAKING] next to all breaking changes
• [x] I have updated the documentation if necessary
• [x] If 01_square example was changed, src/lib.rs and README.md were updated