vbo: optimize some display list drawing (v2)

vbo: optimize some display list drawing (v2)

The vbo_save_vertex_list structure records one or more glBegin/End
primitives which all have the same vertex format.

To draw these primitives, we setup the vertex array state, then
issue the drawing command.  Before, the 'start' vertex was typically
zero and we used the vertex array pointer to indicate where the
vertex data starts.

This patch checks if the vertex buffer offset is an exact multiple of
the vertex size.  If so, that means we can use zero-based vertex array
pointers and use the draw's start value to indicate where the vertex
data starts.

This means a series of display list drawing commands may have
identical vertex array state.  This will get filtered out by the
Gallium CSO module so we can issue a tight series of drawing commands
without state changes to the device.

Note that this also works for a series of glCallList commands (not
just one list that contains multiple glBegin/End pairs).

No Piglit or conform changes.

v2: minor fixes suggested by Ian.

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