u_primconvert: Copy min/max_index from the original primitive.

u_primconvert: Copy min/max_index from the original primitive.

These values are supposed to be the minimum/maximum index values used to
read from the vertex buffers.  This code either copies index values out of
the old IB (so, same min/max as the original draw call), or generates a
new IB (using index values between the start and the start + count of the
old array draw info, which just happens to be what min/max_index are set
to by st_draw.c).

We were incorrectly setting the max_index in the
converting-from-glDrawArrays case to the start vertex plus the number of
vertices generated in the new IB, which broke QUADS primitive conversion
on VC4 (where max_index really has to be correct, or the kernel might
reject your draw call due to buffer overflow).

Reviewed-by: Rob Clark <robclark@freedesktop.org> (from verbal description
             of the patch)