Avatars that are semi-transparent are a problem when viewing a thread,
as the line that connects different statuses in the same thread is drawn
underneath the avatar and is visible.
Fix this with a CompositeWithOpaqueBackground Glide transformation that:
1. Extracts the alpha channel from the avatar image
2. Converts the alpha to a 1bpp mask
3. Draws that mask on a new bitmap, with the appropriate background
colour
4. Draws the original bitmap on top of that
So any partially transparent areas of the original image are drawn over
a solid background colour, so anything drawn under them will not appear.
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