splash: fix background drawing issue

- fix horizontal line drawing issue in the bottom
  right corner if the size of background image and
  screen is similar which caused by gdkpixbuf
- add test code for splash utility functions

tower: https://tower.im/projects/1c0cd0c59ef941298c6e6b2ba6833b91/todos/1e45cb5d4c5940f0b960edba631a621a/

Change-Id: I620058cc14fa5eed965e546d48171049a56632f6