pad 1 or 2 channel images saved in gui window .stack

[mrariden]

2D images loaded into the 2D CP GUI would load correctly but were incorrectly padded to 1 channel instead of 3 channels. Previously, 2 channel images were correctly padded but not 1. The input to the network requires 3 channels.

The GUI currently shows red instead of gray because this line doesn't fire:

cellpose/cellpose/gui/io.py

Line 124 in 9947b9a

if len(np.unique(image[..., 1:])) == 1:

The root cause of this is the different behavior of transforms.convert_image() which guaranteed a 3 channel image.

[carsen-stringer]

To reduce memory I didn't pad the images, could we just make the gray channel open instead of red?