This commit fixes a pretty long-standing bug in the X11 clipboard code
which caused selection target requests (and thus pasting) to sometimes
fail on certain softwares like Firefox.
The problem lied in the fact that the property format used when setting
the supported target list was dependent on the original request, when
it should always be XA_ATOM (a list of Atom), this can be seen in other
implementations, like GLFW.
This commit also cleans up the surrounding code, by using m_atom to
access atoms instead of redeclaring them in the function, which also
clears up a few instance where the difference between `xse->target`
(the request target) and `target` (the TARGETS Atom) was really unclear.
Pull Request: https://projects.blender.org/blender/blender/pulls/131507
720fc44ca3