Cosmetic updates to glFVarViewer

- apply image shader on background (look consistent with glViewer)
- fix retina display bug
Cosmetic update to glEvalLimit

- add image shader to fill background gradient
Cosmetic changes to glShareTopology

- add image shader to fill baground gradient
- fix retina display bug
Cosmetic update to glPaintTest code example

- added image shader to fill in background gradient
- fix retina display bug