When rendering progressively, make sure that any previously visible area that
is still visible is updated at the same time. This helps maintain visual
coherency on pages that invalidate previously visible areas while scrolling,
and when losing layers between updates.
This supersedes the previous method of only doing progressive updates while
scrolling.
Rather than discarding uploaded tiles once they fall outside of the valid
region, shuffle them off into a separate tile-store with all the metadata they
need to render them, and render them in the areas that our current valid
region doesn't cover in TiledThebesLayerOGL::RenderLayer.
Rather than discarding uploaded tiles once they fall outside of the valid
region, shuffle them off into a separate tile-store with all the metadata they
need to render them, and render them in the areas that our current valid
region doesn't cover in TiledThebesLayerOGL::RenderLayer.
