Add unit tests for image i/o.
Add unit tests for image i/o.

For now, we're just testing that we get back the pixels we wrote out, in
the same order.

Note that everything but EXR has a (shouldn't be necessary) y flip enabled
so that the tests will pass. Further discussion and a sketch of a solution
in issue #74.

nispaur/pbrt-v3,nyue/pbrt-v3,ambakshi/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,ambakshi/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,ambakshi/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3