Fix image comparison

Fixes the compare_image RMS calculation algorithm, so that it computes the RMS of the difference between corresponding pixels, as opposed to the RMS of the histograms between the two images.

See discussion on [Issue 1287](https://github.com/matplotlib/matplotlib/issues/1287).

**Note:** This is *not yet ready* to merge, since it breaks a lot of tests. Some negotiation is required to figure out whether to update the expected output for each test, or bump up the tolerance.