FontBase, TextMesh improvements
DeferredLoader fixes

Deferred loads that trigger multiple other deferred asset loads must
hold a BackingAssetLink for those dependent assets that finish loading
to prevent them from being pruned if the rest of the loading process
takes a long time.  This is now handled by using ConstLinkedAssetPtr<T>
instead of ConstAHandle<T>.

Also fixed: TimedBackingAsset only resets the last usage time in
decrementLinks_() if the asset no longer has any links backing it.
renamed LinkedAssetPtr -> LinkedAHandle
fixed linked_asset_ptr filenames