fix(thumbnails): respect the requested height in libvips builds

[JammingBen]

The libvips generator passed 0 as the target height to vips_thumbnail, so the value was rejected and dropped.

As a result, a preview of a 4000x5000 portrait image requested with e.g. x=500&y=500 returned a ...x1920 image instead of a ...x1080 one, which would be the next correct size in the pre-defined resolutions list. This was due to the missing height, so it used the width (=1920) to determine the longest side.

This also aligns it with the non-libvips behavior.

fixes #3376

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 duplication

Metric	Results
Duplication	0

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🟢 Coverage ∅ diff coverage · +0.00% coverage variation

Metric	Results
Coverage variation	✅ +0.00% coverage variation (-1.00%)
Diff coverage	✅ ∅ diff coverage

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Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (b62af81)	84332	19631	23.28%
Head commit (d562d44)	84332 (+0)	19631 (+0)	23.28% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3377)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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