Replaced the Image::generateMipmap switch statements with function calls to get the mip generation function.

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@2319 736b8ea6-26fd-11df-bfd4-992fa37f6226