TRAC #12245
This ensures that the shader will fail to compile when using VTF when it's not supported.
Previously an error was generated only at link time and it was not very descriptive.
Signed-off-by: Daniel Koch

Author:    Nicolas Capens

git-svn-id: https://angleproject.googlecode.com/svn/trunk@433 736b8ea6-26fd-11df-bfd4-992fa37f6226