Before calling _mesa_create_context(), initialize a dd_function_table struct

Before calling _mesa_create_context(), initialize a dd_function_table struct
by calling _mesa_init_driver_functions() and then plugging in the driver-
specific functions.
In particular, make sure ctx->Driver.NewTextureObject points to the
appropriate driver function so that _all_ texture objects are augmented
with the driver-specific data.
Put in a bunch of assertions in the texture-related driver functions that
texObj->DriverData is valid.  Remove old dead code in near future.