Blender Git Loki

Timeline addition: Display cached frames

This started off doing pointcache debugging but it's also very useful for users too.
Previously it was very hard to see the state of the system when you're working caches
such as physics point cache - is it baked? which frames are cached? is it out of date?

Now, for better feedback, cached frames are drawn for the active object at the bottom
of the timeline - a semitransparent area shows the entire cache extents, and more
solid blocks on top show the frames that are cached. Darker versions indicate it's
using a disk cache.

It can be disabled in general in the timeline View -> Caches menu, or by each individual
system that can be shown.

There's still a bit to do on this, behaviour needs to be clarified still eg. deciding what
shows when it's out of date, or when it's been played back but not cached, etc. etc.
Part of this is due to a lack of definition in the point cache system itself, so we should
try and clean up/clarify this behaviour and what it means to users, at the same time.

Also would be interested in extending this to other caches such as fluid cache,
sequencer memory cache etc. in the future, too.