Add an skp loader to CanvasKit viewer

The new loader works by checking for a "slide" flag, and if it ends in
".skp", then we treat the slide name as a URL and try to pull it in with
an HTTP request and parse it as an SkPicture.

It is the user's responsibility to copy or link skps into their
canvaskit server directory.

Change-Id: Iaafa84300d36d2d5a0bb29c47761ec67076c0f50
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/292204
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
5 files changed