Revert "Enable hidpi rendering of titlebar buttons when the theme file wants it."

This reverts commit 50e84af4d81c3b5dbe5b79357dae98643788a3c0.
Simpler implementation of hidpi support for metacity theme images.

No modification of themes needed - images are checked for their natural
size, then reloaded at the proper size * scale value.  This ensures svg
files get loaded to a fairly sane size before being pixbuf scaled.