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

[ForNeVeR]

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.