The code that draws the picture knows more about the picture (and the future)
than we do. This patch gives them a way to hint us about what's going to
happen with the picture in the future.
After an application is suspended and the GrContext associated with the
Flutter view is destroyed, the raster cache still contains images tied
to the defunct context. The SkyShell process will crash if these images
are used after the application resumes.
