Apps can load cube faces in an arbitrary order, so validating texsubimage based on the format of only the first face won't work.

BUG=149
TEST=

Review URL: http://codereview.appspot.com/4445085

git-svn-id: https://angleproject.googlecode.com/svn/trunk@629 736b8ea6-26fd-11df-bfd4-992fa37f6226