Fix vertex cache bug that allows multiple vertices to fall into the same slot.

Fix vertex cache bug that allows multiple vertices to fall into the same slot.

Need to set the slot's bit in draw->vcache.referenced even when there was a
cache hit since flushing the primitive buffer will have cleared the bitfield
but not the cache's vertex indexes.

Fixes a bug found when drawing long triangle fans but could be hit by other
prim types as well.

An alternate fix would be to call draw_vertex_cache_invalidate() from
draw_vertex_cache_unreference().