Allow full visibility of transparent textures

[JannikGM]

We have a couple of RGBA textures which have alpha = 0.0 for the majority of the pixels, but it contains important information in the RGB channels. It's currently impossible to tell what the contents of the pixels are, because only the checkerboard pattern shows in the sidebar.

The texture viewer should separate RGB and Alpha, and display RGB for the colors, and alpha as grayscale.
A checkbox to display the texture opaque, or to mask/unmask channel visibility could also work.