PHP 8.1 | MigrationGuide/New constants: add missing constants [2]

> GD:
> * Avif support is now available through the `imagecreatefromavif()` and
>     `imageavif()` functions, if libgd has been built with avif support.

While not mentioned in the changelog entry, the commit to PHP does contain a new constant declaration...

Refs:
* https://github.com/php/php-src/blob/f67986a9218f4889d9352a87c29337a5b6eaa4bd/UPGRADING#L245-L247
* php/php-src#7026
* php/php-src@81f6d36#diff-00d1efef2247b288c86a6c3bfefac111a4774fbc5453fdc02dcf36c4a23da283R373

> GD:
> * `imagewebp()` can do lossless WebP encoding by passing `IMG_WEBP_LOSSLESS` as
>    quality. This constant is only defined, if a libgd is used which supports
>    lossless WebP encoding.

Refs:
* https://github.com/php/php-src/blob/3a71fcf5caf042a4ce8a586a6b554fd70432e1e2/UPGRADING#L568-L571
* php/php-src#7348
* php/php-src@eb6c9eb

Author: jrfnl

appendices/migration81/constants.xml

@@ -33,6 +33,19 @@
   </itemizedlist>
  </sect2>
 
+ <sect2 xml:id="migration81.constants.gd">
+  <title>GD</title>
+
+  <itemizedlist>
+   <listitem>
+    <simpara><constant>IMG_AVIF</constant></simpara>
+   </listitem>
+   <listitem>
+    <simpara><constant>IMG_WEBP_LOSSLESS</constant></simpara>
+   </listitem>
+  </itemizedlist>
+ </sect2>
+
  <sect2 xml:id="migration81.constants.mysqli">
   <title>MySQLi</title>