Added an abstract BufferStorage::copyData method and implemented it for BufferStorage9 and BufferStorage11.

TRAC #22811

Signed-off-by: Jamie Madill
Signed-off-by: Shannon Woods
Author: Geoff Lang

git-svn-id: https://angleproject.googlecode.com/svn/branches/es3proto@2132 736b8ea6-26fd-11df-bfd4-992fa37f6226