gdImageOpenPolygon() has been introduced with líbgd 2.0.29, so we finally
add a PHP binding for it, and port the respective libgd functionality to
PHP's bundled libgd.
Instead of rolling our own in the bundled libgd, we use libgd's anti-aliased
drawing API. This way imageantialias() is also available, when built against
a system libgd.
We apply the law `(b**r)**s == b**(r*s)` which holds for all non-negative b
and positive r,s, so a single pow() suffices. Furthermore, we precompute the
gamma, so the refactored code is simpler and faster.
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 GD image format is able to handle truecolor images as of libgd 2.0.12
(<https://github.com/libgd/libgd/blob/gd-2.2.3/src/gd_gd.c#L31-L33>).
Therefore we don't need the potentially lossy and time consuming palette
conversion.
This way, imagegd() can also be used to export raw truecolor image data.
UTF-8 font file names are not yet supported on Windows[1], so we're skipping
the respective tests on this platform, but enable them on other platforms.
[1] <http://news.php.net/php.internals.win/1142>
Firstly, there is no fundamental difference between running the tests on
freetype before and after 2.4.10, so we re-join the respective test cases.
Secondly, there are some minor differences in the results depending on
architecture, freetype versions and config options, so we cater to that
by allowing small deviations from the expectations.
Apparently, different FreeType versions render the string differently, so
we have to cater to these slight differences. Unfortunately, the testing
framework doesn't yet offer a clean API for this, so we work around the
limitations by parsing and checking the output of test_image_equals_file().
