* [fuchsia] fix Rasterizer::ScreenshotLayerTreeAsPicture on Fuchsia
* [fuchsia] Add shell test for skp based shader warmup
* [fuchsia] use custom image serialization when writing out SKP's
This change adds custom image serialization functions for skp serialization
which write out onlythe image metadata and not the contents of the
images themselves. This allows SKP's to be used for shader warmup with
a significantly reduced disk space footprint.
