This patch enables image user nodes to display the alpha mode selector.

This patch does not change the Image Texture Geometry Node.

I removed the function image_has_alpha in favor of using BKE_image_has_alpha. I removed this function for two reasons. First, image_buttons.c does not seem like the proper place to define this kind of function. Second, image_has_alpha did not check if the image actually had an alpha channel. The function just checked if the image type was capable of supporting an alpha channel.