* lisp/htmlfontify.el: Fix misuses of `nconc`

* lisp/htmlfontify.el: Fix misuses of `nconc`

(hfy-face-to-style-i): `this` is not known to be fresh.
(hfy--size-to-int): Rename from `hfy--size-to-int` and return just the
integer without wrapping it in a list.
(hfy-flatten-style): Avoid O(n²) problems.  Use `float`.