more opinionated file loading in fm

Only try to parse .skp files as SkPictures, only .svg
files as SVG, and only try to parse any given file once.

This cleans up error logging from, e.g., trying to
parse a .jpg as an .svg.  Sometimes even asserts.

Change-Id: Ibfab76e201eb273107b96fcbd3af94d397862291
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204882
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed