Validate cube texsubimage based on current face

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
3 files changed