Gathers the primitive's dynamic mesh elements. This will only be called if GetViewRelevance declares dynamic relevance. This is called from the rendering thread for each set of views that might be rendered. Game thread state like UObjects must have their properties mirrored on the proxy to avoid race conditions. The rendering thread must not dereference UObjects. The gathered mesh elements will be used multiple times, any memory referenced must last as long as the Collector (eg no stack memory should be referenced). This function should not modify the proxy but simply collect a description of things to render. Updates to the proxy need to be pushed from game thread or external events.