vbo/dlist: add vertices to incomplete primitives

vbo/dlist: add vertices to incomplete primitives

If a primitive is added with missing vertices, this will shift all
the vertices and produce incorrect rendering.

In issue #5786 the app uses GL_LINE_STRIPS with a single vertex.

Adding extra vertices can make the initial estimation for the index
buffer size incorrect, so this buffer can now be growed if needed.

Fixes: ac3d4c7635b ("vbo/dlist: convert LINE_STRIPS to LINES")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5786
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14243>