Support passing single SKP to get_images_from_skps

In addition to allowing the user to supply a directory, allow them
to supply a file. Simplifies my typical use case of testing a single
file.

Change-Id: I4f268cfb33fc70ff3121135941693023b6840cd3
Reviewed-on: https://skia-review.googlesource.com/18586
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
1 file changed