tests: make vertex-clip use WESTON_EXPORT_FOR_TESTS

This is probably the simplest case to demonstrate how to use
WESTON_EXPORT_FOR_TESTS.

Previously, vertex-clip test re-built vertex-clipping.c for itself. Now
it directly links in gl-renderer.so instead as that is where
vexter-clipping.c gets built into for actual use. This probably will not
work for any installed program, but luckily tests are never installed,
so Meson makes sure the DSO is found.

Unfortunately we cannot remove the definition of dep_vertex_clipping
yet, because clients/cliptest.c needs it.

This makes vertex-clip test depend on GL-renderer, but that is where the
code is really used.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>