This seems like a good application of the Approval Tests framework or something like it.
As stated in the comments, you are still going to have an issue with false positives, if you happen to approve bad output, but this will at LEAST tell you when output has changed significantly.
Since you are using OpenGL, I'm assuming that approvals won't work for you directly, but the idea is sound. Just check the file hash and if it is different, show the failure in an appropriate diff viewer (like an image diff program). If you approve of the new version, update the "approved" file to match the new result.