gn: fix fiddle

  - __SK_FORCE_IMAGE_DECODER_LINKING makes sure the PNG encoder is linked
  - updated path to make working with sample draw.cpp easier.

No public API changes.
TBR=reed@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2202353002

Review-Url: https://codereview.chromium.org/2202353002
4 files changed