Read IM and TIFF images as RGB, rather than RGBX

[radarhere]

Helps #2228

Rather than reading some IM and TIFF images into the "experimental" mode RGBX, load them as RGB.

This idea is already employed in WebP.

Pillow/src/PIL/WebPImagePlugin.py

Line 80 in f8160b8

self._mode = "RGB" if mode == "RGBX" else mode

This is not removing any functionality of of the RGBX mode, just changing the mode that images are when they are read.