This patch should contain all the code we need to run Flutter on
Fuchsia's software framebuffer (without text). I haven't actually tried
running the code, so I'm sure it doesn't work yet.
Also, rename SkyViewClient to RuntimeDelegate. These names are more
sensible.
This patch also cleans up the RuntimeDelegate a bit, for example by
removing support for flushing real-time events, which aren't used.