Blender Git Loki

Fix T74586: Image Editor Uses Invalid Display Channels

When using the image editor the display channels attribute can become
invalid when selecting another image/buffer. This patch will check what
display channels are valid and when an invalid channel is selected it
will fall back to the color channel.

To de-duplicate the code it also introduces a
`ED_space_image_get_display_channel_mask` function that will determine
the valid bitflags for the display channel of a given `ImBuf`.