Update wmtssource.js

- "duplicated" style parameter is not supported by Intergraph GeoMedia
- Uppercase Parameter is needed by ARCGis 
- are there other constraints?
Move font-family customization to layout.css
A bit more static image doc
Similar detail for all crossOrigin options
Make stability checkbox sticky
Merge pull request #2939 from tschaub/sticky-stability

Make stability checkbox sticky.
Merge pull request #2938 from tschaub/image-static-docs

Additional image static docs.
Merge pull request #2896 from kalinbas/patch-1

Remove duplicated style parameter
Merge pull request #2895 from fredj/remove-font-family

Move font-family customization to layout.css
Fix typo in selector
Merge pull request #2959 from ahocevar/unstable-doc-fix

Fix typo in selector