Created new helper functions for converting texture formats, loading images and generating mipmaps."

TRAC #22972

Signed-off-by: Jamie Madill
Signed-off-by: Nicolas Capens
Author: Geoff Lang

git-svn-id: https://angleproject.googlecode.com/svn/branches/es3proto@2313 736b8ea6-26fd-11df-bfd4-992fa37f6226