[Fixes #11554] Implement WMTS backgrounds for thumbnails (only 3857 tilematrix sets)

[giohappy]

Checklist

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[codecov]

Codecov Report

Merging #11555 (b0fa447) into master (9f88645) will decrease coverage by 0.23%.
The diff coverage is 6.79%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11555      +/-   ##
==========================================
- Coverage   62.34%   62.12%   -0.23%     
==========================================
  Files         868      869       +1     
  Lines       51786    51992     +206     
  Branches     6475     6491      +16     
==========================================
+ Hits        32287    32301      +14     
- Misses      17944    18136     +192     
  Partials     1555     1555