Propagating lossless conversion from libgd to our bundled gd.
Changing "quantization" to "quality" as in libgd.
Adding test.
IMG_WEBP_LOSSLESS is only defined, if lossless WebP encoding is
supported by the libgd used.
Closes GH-7348.
* --with-webp-dir becomes --with-webp
* --with-jpeg-dir becomes --with-jpeg
* --with-png-dir is removed. libpng is required.
* --with-zlib-dir is removed. zlib is required.
* --with-xpm-dir becomes --with-xpm.
We also enable --with-webp on Travis.
ext/gd/libgd/gd_webp.c has been taken from libgd[1]. Mainly, gd_error(X) has
been subsituted by zend_error(E_ERROR, X) and BGD_DECLARE(X) by X. Further
modifications are obvious from the diff.
All GD tests are passing, what raises hope, but we need more WebP tests,
anyway.
[1] <7ec030c4f1/src/gd_webp.c>
