windows.txt: analyze the last two values of the PANE field

In the `windows.txt` configuration file, most `PANE` items include four values, like `PANE 1 2 3 4` (which are actually x₀, y₀, x₁, y₁). But for some items, such as `BC_TrnStkC`, there are two unknown additional numbers there (`-4 0` in that case).

WE should figure out if these values mean anything.

See `TODO[#31]` in the code for this issue.