i965: Add a mechanism for sending native primitives into the driver

i965: Add a mechanism for sending native primitives into the driver

The brw_draw_prims() function is the draw entry point into the driver,
and takes struct _mesa_prim for input.  We want to be able to feed
native primitives into the driver, and to that end we introduce
BRW_PRIM_OFFSET, which lets use describe geometry using the native
GEN primitive types.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>