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.
The bundled GD test suites makes heavy use of md5() to verify the result
of drawing operations. This leads to fragile tests (even a slight change
in a PNG header would cause failure, and of course there is the
possibility of collisions), and even worse, eventual test failures are
rather unrevealing.
Therefore we replace all md5() verification with a simplistic
test_image_equals_file(), which is basically a simplified port of libgd's
gdTestImageCompareToFile(), adapted to the needs of PHPTs.
In the long run better tests helpers should be introduced (see also
<http://news.php.net/php.internals/94081>), but for now this solution
is preferable over the former.
(cherry picked from commit 24f9e96792)
