jpegr: initialize legacy gain map output

[gregbenz]

What changed

• Correctly configure the gain-map output in the legacy decodeJPEGR() wrapper.
• Add a test that requests the primary image, gain map, and metadata together.

Why

When a caller asks for both the decoded image and its gain map, the wrapper puts the
gain-map settings on the wrong output and passes an uninitialized descriptor for the
gain map. This causes the decode to fail. The fix configures the correct output so both
images are returned successfully.

Testing

• Added JpegRTest.DecodeLegacyApiWithGainMapOutputKeepsPrimaryDescriptor; it fails on
  current main and passes with this change.
• Full non-HEIF unit suite: 1,291 tests run; 1,067 passed and 224 existing parameter
  combinations skipped.

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