Explicitly use PNG decoder in viewer

This also updates other uses of the PNG decoder to use either
libpng or rust.

Change-Id: I9799e7b1da3b0186d18e7433148f47bee46987e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1140757
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>