ossp-bg.xcf:
o derived from ossp-puzzle
o used for the canvas background at right top position
o generate ossp-bg-light.jpg in Gimp:
  - disable display of "Background Light" and "Background Dark" layers
  - "merge down visible layers" with "expand as necessary"
  - change "opacity" of merged layers to "25.5"
  - enable display of "Background Light" layer
  - use "crop & resize" tool to crop with X=0/Y=100/W=400/H=700
  - save as "ossp-bg-light.jpg" with quality "0.75"
o generate ossp-bg-dark.jpg in Gimp:
  - disable display of "Background Light" and "Background Dark" layers
  - "merge down visible layers" with "expand as necessary"
  - change "opacity" of merged layers to "25.5"
  - enable display of "Background Dark" layer
  - use "crop & resize" tool to crop with X=0/Y=100/W=400/H=100
  - save as "ossp-bg-dark.jpg" with quality "0.75"