Panel size & location bug

[praftery]

I'm guessing you already know about this, but it's possible to specify a panel that extends beyond the wall, causing this:

@thoyt

[praftery]

I see two routes to resolve this out of bounds issue.

1. When a user enters a value that would cause this, we edit the value for the opposing property. For example, starting from a valid scenario in a 10 m wide room:
   xposition = 3
   panel width = 6.
   There's 1 m left over. If the user increases the panel width to 9, we'd move the window over to accommodate it (change xposition to 1). This could be a bit jarring as you'd click on one input and a different input value would change automagically....

2. The alternative is to define it differently, using:
   xcenter (denotes the center of the panel in the surface in the x direction)
   xpct (denotes the percentage of the widest possible panel at that location, given the room width constraint). Maybe a little less intuitive up front (?), but you'd never encounter a case where you'd change one input and another would also change.

I guess option 2 might be better, because none of the inputs would change based on other inputs. Any thoughts before I go one route or the other?