Updated logic to check for external texture object

Since we create the texture object internally as well
the reasonable way to know if the texture object is
set externally is to use an ivar for that purpose which
get set to trun when SetTextureObject is invoked. Checking
via pipeline may result in warnings and that's why its
avoided.
Change-Id: I2908b7e4eebaef66d266affb7e68192c168ad20e