* request_render_task() now returns a handle to a render task
  cache entry, rather than an allocated texture cache item. This
  is more in line with how normal texture cache request work.

* Allows the allocation to happen later in the frame (for example
  if the exact render task size is not yet known).

* Allows storing more information about a cached render task
  than what a texture cache item stores (for example, a specific
  blend mode that is required).

* Allows batching code to cheaply look up a cached render task
  entry by using a freelist instead of a hashmap. This will become
  more important as we start to cache render tasks that have
  larger / more complex cache keys.

* Add an extra type parameter to freelist.rs to allow using a custom
  marker type. This avoids rustc errors related to Send or Sync
  not being implemented on free list handles with !Send or !Sync
  phantom data types.

* Allow the size of a render task to be set to None initially, if
  it is not known.

* Add a new render task call (prepare_for_render) which can be used
  by render tasks to prepare tasks that rely on glyphs and/or images
  in the resource cache to be resolved first. This can also be used
  to set the size of a render task before render target allocation,
  if the render task size is not known during initial visibility pass.