Interpret subfixture windows as relative coordinates

[fwcd]

Currently subfixture windows are interpreted as global coordinates. This is a bit unintuitive, not just because they are named relative_window in the code, but also because it poses a few practical problems:

• If the subfixture is positioned outside the main/parent fixture window, it will effectively not be used at all
• The main fixture still affects the fixture's master (or anything not mapped to a subfixture), so there's a discrepancy that may result in odd lighting effects (color/position not being properly in sync)

This fixes the issue by interpreting the subfixture in the parent fixture's window/coordinate system: