TRAC #19335
Issue=271
Instead, the texture maintains an internal refcount which causes the pointer to behave
kind of like a "weak" shared pointer-- its reference won't prevent the renderbuffer from
being deleted, but if the renderbuffer's refcount drops to zero, the texture should set
its pointer to the renderbuffer to null so that a new one can be created at the next
getRenderbuffer call.

Signed-off-by: Nicolas Capens
Signed-off-by: Daniel Koch

git-svn-id: https://angleproject.googlecode.com/svn/trunk@948 736b8ea6-26fd-11df-bfd4-992fa37f6226