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.

https://github.com/python-pillow/Pillow/blob/f8160b858aebf8b7bffdefba48735feac300a060/src/PIL/WebPImagePlugin.py#L80

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