RendererServices - remove horrible lock every time we did a texture call

This didn't affect SPI's renderer, because these methods were overloaded
in the derived class we use for our internal RS.  But if somebody else was
integrating OSL into a renderer and used the default (base class) texture
functions, they'd run into a serious threading performance issue because
of the lock.  This fixes it by allocating a TextureSystem when the RS is
constructed, then just calls through the pointer without any locks for
individual texture function calls.