gainmap: validate destination layout before applying gain map

[gregbenz]

Summary

• Require the destination width and height passed to applyGainMap() to match the SDR input/output dimensions.
• Validate the packed destination stride against the width actually processed by the write loop.
• Document the destination layout contract and add regression coverage for mismatched dimensions, insufficient stride, and valid row padding.

Why

applyGainMap() processes sdr_intent->w by sdr_intent->h pixels, but the existing validation only checks whether the packed stride is at least dest->w. A destination descriptor with smaller logical dimensions can therefore pass validation even though processing and raw-pointer writes continue over the full SDR extent.

Related: #367. This PR addresses the same destination-buffer validation issue by rejecting inconsistent destination dimensions and stride before CPU or GLES dispatch. Raw plane writes do not throw std::out_of_range, so validation is performed at the descriptor boundary.

Compatibility

The destination's logical dimensions must describe the generated output, so dest->w and dest->h must equal sdr_intent->w and sdr_intent->h. Callers may still use row padding by setting the packed destination stride greater than or equal to the output width.

The descriptor does not expose allocation capacity, so callers remain responsible for providing storage large enough for the declared stride and height.

Testing

• JpegRTest.ApplyGainMapInvalidArgs
• Complete default unit-test suite
• Complete AddressSanitizer unit-test suite
• Complete AVIF-enabled unit-test suite
• UltraHdrApiTest.Avif* (3 tests)
• git diff --check upstream/main...HEAD

All checks pass.

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