vbo: fix VBO optimization regression

vbo: fix VBO optimization regression

The optimization in change 8e4efdc895ea ("vbo: optimize some display
list drawing") missed the loopback case.  This is used when the
glBegin/End primitive doesn't have a uniform set of vertex attributes.
The new Piglit gl-1.0-dlist-materials test hits this.

So check the aligned_vertex_buffer_offset(list) value and adjust the
buffer offset accordingly.

We also need to remove the 'start == 0' assertion in the loopback
code since it no longer applies.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>