Add support for reading X11 cursors

It is an uncompressed format using alpha-premultiplied ARGB32, and it supports animations:
https://www.x.org/releases/current/doc/man/man3/Xcursor.3.xhtml

Reading in only one nominal size would already be useful, e.g., the first contiguous stream of equivalent dimensions in the file.

I might try to implement it myself. I suspect the greatest issue will be handling seeking, since the format uses byte offsets. They are always little-endian, because that is what libXcursor does.