Track content_age of LayerBuffer in the struct itself

Instead of relying on the TileGrid to track content age, just store the
information in the LayerBuffer itself. This means that we must pass it
with BufferRequests as well, so that the renderer can populate th
LayerBuffer properly.
Add a method marking all tiles as having changed contents
Merge pull request #97 from mrobinson/pinch-zoom-delay

Allow marking all layers as having changed contents