Merge.
Introduces a new Display Object flag to keep track of dirty descendents.

This makes it possible to distinguish between objects with dirty properties and those that have descendents with dirty properties. This allows us to reduce the number of objects that we remote, since we don't have to remote those that don't have dirty properties.
Don't make graphics dirty just because its content has changed.
Don't invalidate GFX stage for video elements that are composited with DOM elements. This avoids repaints triggered by video updates.
Sample Video subtitles.
Merge.
Merge pull request #1927 from mbebenita/fb1

Fb1