There were two problems here:
- libdecor_frame_get_content_* is not available in LIBDECOR v0.1.0.
- These functions aren't exposed by <libdecor.h>,
they're only exposed by `libdecor-plugin.h`
(intended for plug-ins that implement window decorations).
Resolve by storing the last applied size from LIBDECOR for reuse.