Don't invert the alpha channel in the Invert Image node

ImageInvert applied 1.0 - image across the whole tensor, so a 4 channel
RGBA image came back with its transparency flipped: opaque pixels became
transparent and vice versa. Alpha stores coverage, not color, so leave it
alone and only invert RGB. Matches GIMP, ImageMagick and Blender, and the
existing srgb_to_linear/hlg_to_linear handling in nodes_images.py.

Reported in #5610.

nodes.py

@@ -1947,6 +1947,8 @@ class ImageInvert:
 
     def invert(self, image):
         s = 1.0 - image
+        if image.shape[-1] == 4:  # alpha stores transparency, not color
+            s[..., 3] = image[..., 3]
         return (s,)
 
 class ImageBatch: