R1:c2856b479fbd

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…