llvmpipe: bin state-change commands

llvmpipe: bin state-change commands

Previously, each triangle had a pointer to the state to use for shading.
Now we insert state-change commands into the bins.  When we execute one
of those commands we just update a 'current state' pointer and use that
pointer when calling the jit shader.

When inserting state-change commands into a bin we check if the previous
command was also a state-change command and simply replace it.  This
avoids accumulating useless/redundant state-change commands.