vbo: Make vbo_sw_primitive_restart optionally count primitives.

This will be necessary in order to get the i965 back-end to produce
the correct value of gl_PrimitiveIDIn when using software primitive
restart.
!UPSTREAM: i965/gs: Fix gl_PrimitiveIDIn when using SW primitive restart.

TODO: We really ought to set _NEW_PROGRAM_CONSTANTS only when software
primitive restart is in use.