Omits dirty-rect processing on server side in streaming mode.

To improve the server rendering performance.
Use cached _compositeTransform if possible.

This reverts 64324dfe332fb059feef1ffd2ff6d0c9596fa0e0.
Note the cached _compositeTransform is invalidated whenever the node gets dirty (in DisplayNode._setDirtyRect).
Merge pull request #51 from mootoh/optimize/omit_dirty_rect_on_server

Omits dirty-rect processing on server side in streaming mode.