mesa: Make MultiDrawElements submit multiple primitives at once.

mesa: Make MultiDrawElements submit multiple primitives at once.

Previously, MultiDrawElements just called DrawElements a bunch of times.
By sending several primitives down the pipeline at once, we avoid a bunch
of validation.  On my GL demo, this improves fps by 2.5% (+/- .41%) and
reduces CPU usage by 70.5% (+/- 2.9%) (n=3).

Reviewed by: Ian Romanick <ian.d.romanick@intel.com>