Fix PHP 8.1 "implicit conversion from float to int" exception on certain avatar colors which blocks rendering user pages

Summary:
Since PHP 8.1, conversion from float to integer should be explicit.

https://wiki.php.net/rfc/implicit-float-int-deprecate

According to `phlog`, the alpha channel value of the automatically created user
avatar image for a new user account sometimes is a float.

Thus always `round()` the alpha channel value to be an integer.

Closes T15375

Test Plan: Applied this change, created five user accounts via http://phorge.localhost/people/new/standard/ , and each resulting alpha value in `PhabricatorFilesComposeAvatarBuiltinFile.php` was an integer according to `phlog` (see corresponding Maniphest task). Rendering of each new user page always succeeded.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15375

Differential Revision: https://we.phorge.it/D25209

src/applications/files/builtin/PhabricatorFilesComposeAvatarBuiltinFile.php

@@ -183,12 +183,21 @@ final class PhabricatorFilesComposeAvatarBuiltinFile
       'image/png');
   }
 
+  /**
+   * Convert a color from RGBA to a value usable in PHP-GD.
+   * Each RGB color should be expressed as an integer from 0 to 255.
+   * The Alpha Channel should be expressed as a float from 0 to 1.
+   * @param array $rgba array( int Red, int Green, int Blue, float Alpha )
+   * @return int
+   */
   private static function rgba2gd($rgba) {
+    // When working with a truecolor image, we can use bitwise operations
+    // https://www.php.net/manual/en/function.imagecolorallocate.php#49168
     $r = $rgba[0];
     $g = $rgba[1];
     $b = $rgba[2];
     $a = $rgba[3];
-    $a = (1 - $a) * 255;
+    $a = round(((1 - $a) * 255), 0);
     return ($a << 24) | ($r << 16) | ($g << 8) | $b;
   }