Feature/#111 Automate Reference Images

A new method REQUIRE_REFERENCE_IMAGE_SIMILARITY has been added to test_utilities::OpenGLBufferComparison that compares an image buffer against a reference image path. If the similarity is too low, or if the reference image does not exist, the buffer is written to disk and the user is prompted to compare it manually against the reference image. If the test image is actually correct, and the reference image is wrong/outdated/non-existent, the user can write "(over)write" to the console, and the reference image is updated. For tests and on the build server, std::cin can be filled with the preferred user response. On the build server, an empty std::cin will result in an empty-string response (-> no (over)write and therefore test fail).

Assigned to @jwendt to sign off on the CMake-related changes. Requested additional reviewers: @ademiralp, @mbellgardt.

Closes #111