Fix tile rendering

[clarasb]

This PR is a follow-up to #1216.

#1216 improved the tile indexing step by using nearest-pixel rounding when mapping tile pixels to dataset pixels. This fixed the actual tile shift, because target pixels are now sampled from the nearest source pixel instead of being consistently biased toward one side.

However, the source subset used for rendering was still computed too tightly. The subset bbox describes the transformed positions of the target tile pixel centers, but nearest-pixel rounding may select a source pixel whose center lies just outside that bbox.

source pixel centers:
      x        x        x        x
      |--------|--------|--------|

transformed target bbox:
           |----------------|
           ^                ^
   nearest source pixel     
   may lie outside bbox     

If such an edge source pixel was not included in the loaded subset, it was treated as invalid and rendered as missing or transparent data. This PR fixes this behaviour by enlarging the source subset now using the dataset’s own pixel size, including the half-pixel margin required by nearest-pixel rounding.

Example: xcube serve -vvv -c examples/serve/panels-demo/config.yaml

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.60%. Comparing base (2a384fc) to head (036ba4d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1234      +/-   ##
==========================================
- Coverage   89.61%   89.60%   -0.01%     
==========================================
  Files         280      280              
  Lines       21694    21700       +6     
==========================================
+ Hits        19440    19444       +4     
- Misses       2254     2256       +2     

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[TonioF]

Just one comment, otherwise it looks good to me.

[TonioF]

Suggested change

	reducing visual offsets between rendered map tiles and dataset coordinates. (#1216, #1234)
	Reducing visual offsets between rendered map tiles and dataset coordinates. (#1216, #1234)