fix: qwen2.5vl, qwen3vl composite image (#12841)

this change fixes images with an alpha channel by overlaying the image
onto a white background

model/models/qwen25vl/process_image.go

@@ -79,6 +79,8 @@ type Grid struct {
 }
 
 func (p *ImageProcessor) ProcessImage(img image.Image) ([]float32, *Grid, error) {
+	img = imageproc.Composite(img)
+
 	origWidth := img.Bounds().Dx()
 	origHeight := img.Bounds().Dy()
 

model/models/qwen3vl/imageprocessor.go

@@ -83,6 +83,8 @@ type Grid struct {
 }
 
 func (p *ImageProcessor) ProcessImage(ctx ml.Context, img image.Image) (ml.Tensor, *Grid, error) {
+	img = imageproc.Composite(img)
+
 	origWidth := img.Bounds().Dx()
 	origHeight := img.Bounds().Dy()