various fixes vor php 8.1 compatibility

Cherry-picked from https://git-gitea.tt-rss.org/fox/tt-rss/pulls/56,
while excluding changes in vendor/ (causes compatiblity issues with
PHP<8 [1]) and strftime-related ones (already re-applied in
https://git-gitea.tt-rss.org/fox/tt-rss/pulls/66).

[1] https://community.tt-rss.org/t/support-for-php-8-1/5089/9

lib/jimIcon.php

@@ -31,7 +31,7 @@ class jimIcon {
                 }
                 if ($a != 127)
                         $this->all_transaprent = 0;
-                return imagecolorallocatealpha($img, $r, $g, $b, $a);
+                return imagecolorallocatealpha($img, $r, $g, $b, (int) $a);
         }
 
         // Given a string with the contents of an .ICO,