Re-submit: Fix: Proper RGBA -> RGB conversion for PIL images. (#18569)

Signed-off-by: Chenheli Hua <huachenheli@outlook.com>

tests/multimodal/test_utils.py

@@ -10,6 +10,7 @@ import numpy as np
 import pytest
 from PIL import Image, ImageChops
 
+from vllm.multimodal.image import convert_image_mode
 from vllm.multimodal.inputs import PlaceholderRange
 from vllm.multimodal.utils import (MediaConnector,
                                    merge_and_sort_multimodal_metadata)
@@ -53,7 +54,7 @@ def get_supported_suffixes() -> tuple[str, ...]:
 
 
 def _image_equals(a: Image.Image, b: Image.Image) -> bool:
-    return (np.asarray(a) == np.asarray(b.convert(a.mode))).all()
+    return (np.asarray(a) == np.asarray(convert_image_mode(b, a.mode))).all()
 
 
 @pytest.mark.asyncio