Removed curly brace array access for > PHP 7.4 use

[mattkirwan]

As outlined in the RFC - https://wiki.php.net/rfc/deprecate_curly_braces_array_access

array access via curly braces is now deprecated.

And for people searching this issue in the future:

ErrorException: Array and string offset access syntax with curly braces is deprecated in AnimGif.php:206

[yannickl88]

I ran into the same issue, would be great if this could be merged

[VarunAgw]

@yannickl88 Meanwhile you can get it from https://github.com/mattkirwan/AnimGif/tree/patch-1