Check for the existence of specific functions instead of just checking
to see if GD is bundled.  This provides granular support in more cases.
Resolves http://dev.kohanaframework.org/issues/4735
Improve GD function detection
- Create constants for the function names that we're testing
- Move function array declaration outside of foreach()
- Drop the !isset() calls because it was broken, and it's also
  unnecessary since all the values are guaranteed to be initialized.
- Change self:: to Image_GD::
- Restore empty() checks.
Image_GD:: -> static::
Go back to using Image_GD:: for consistency with the rest of the file.

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Revert "Image_GD:: -> static::"

This reverts commit 357410627f9b27e5f77b1f2ade1a57a4a0601de9.
Merge pull request #13 from bharat/3.3/develop

Improve granularity of GD function detection