tnl: Respect `start` when converting indices to GLuint

tnl: Respect `start` when converting indices to GLuint

Prior to commit e99e7aa4c1ddd7b8c2c4388f4f8e4fa1955ca771 (mesa: switch
Draw(Range)Elements(BaseVertex) calls to DrawGallium), the indices
parameter of glDrawElements (an offset into the VBO) was handled by
setting ib->ptr.  With that commit, it instead began binding the
index buffer with offset 0, and adding the offset to draw->start,
which eventually becomes prim->start.

t_draw.c's bind_indices() was trying to convert the relevant section of
the index buffer to GLuints, but was failing to account for start, so
it nabbed the wrong portion of the index buffer.

Fixes: e99e7aa4c1d ("mesa: switch Draw(Range)Elements(BaseVertex) calls to DrawGallium")
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8522>