[canvaskit] Update SKP and RTShader examples

Loading an old SKP stops working after a while, so this changes
it to draw something and then deserialize it immediately.

I also noticed that the CPU backend supports atan, so we can
use a more complete example there.

Change-Id: I70bec69d05184c5ea041b143132ddbbd7f63f004
Bug: skia:12439
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/454456
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed