Fix test_image_equals_file() wrt. palette images

Fix test_image_equals_file() wrt. palette images

The recently introduced test_image_equals_file() doesn't properly work for
palette images, because in this case only the palette indexes are compared,
what can lead to false positives and negatives as shown in the added test.

To fix that we convert palette images to truecolor, what is supposed to be
faster than calling imagecolorsforindex() for each pixel.

We furthermore rely on PHP's refcounting to free unused images; after all,
this is not C.