state_tracker: Initialize the draw context only when needed.

state_tracker: Initialize the draw context only when needed.

It's only used for rarely-used deprecated GL features
(feedback/rasterpos), so we can skip the memory allocation and
initialization for it most of the time.

Saves about 659k (out of 1605k) of maximum memory size according to massif
on simulated vc4 glsl-algebraic-add-add-1

Reviewed-by: Brian Paul <brianp@vmware.com>