perf: cache frame and font metrics

[lingsamuel]

This pr made these changes:

1. Reuse rendered bitmap if the command list has no changes. This can reduce useless re-draw every tick. At worst case it should have same performance just like before.
   Further, we can add another render layer that contains volatile draw calls to ensure the cache are not invalidated by varying data.

2. Add LRU cache for font:measure().

3. Returns nil if the file doesn't exist in d2d.Image.new() calls, instead of raises error.

4. Add some metrics functions.