mesa: swtnl and vbo fixes for regressions due to recent refactors and optimizations
The new primitive merging code and VBO interface changes exposed several major bugs in the TNL module's handling of prim[i].start > 0 and drawing with multiple primitives. There were a few minor bugs in that code as well.
This series fixes all the draw-related regressions on the i915 driver between 20.3 and master.