Update Subs-Graphics.php

I did things differently because of this error:  PHP Fatal error:  Allowed memory size of 33554432 bytes exhausted (tried to allocate 36 bytes) in .../Sources/Subs-Graphics.php on line 300 using SMF 2.0.

Logic:
1.  This functions checks if GD is installed and adds a flag for GD2.  I wanted to get rid of the array created with get_extension_funcs so I replaced it with extension_loaded.  I did not look for a functions_exists GD function for this feature to avoid a fight. Besides extension_loaded seemed to be a good fit.  :0
2.  function_exists is all you need to check for GD2 (ref https://api.drupal.org/api/drupal/modules!system!image.gd.inc/function/image_gd_check_settings/7).  If you look it up you will find that their original code was a slightly improved version of this function.

Signed-off-by: butchs <J.Miller@POBox.com>