Image logging: Use relative_filename where appropriate
Image logging: Use relative_filename where appropriate

If an image is in the test pack then we will print the name relative to
the test-pack root.  This is more useful for logging as it's unambiguous
which image is being used.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester