[Feature]: Add option to generate walls instead of masks when importing UVTT

[kwvanderlinde]

Describe the Problem

The line-of-sight portions of a UVTT are more naturally represented using walls instead of 2-dimensional masks. In complicated situations, e.g., auto-generated caves, there is also a noticable delay when importing the UVTT due to the costly unions that must be done, and this would be avoided by generating walls instead.

The Solution you'd like

Add a preference to allow walls to be generated instead of masks when importing UVTT. There would be three options to choose from:

1. Always generate walls
2. Always generate BL
3. Ask each time

If (3) is selected, a dialog will be shown the user before the file is processed so that the user can control the behaviour on a map-by-map basis.

When walls are requested, they would be generated as follows:

• "line_of_sight" data would result in walls that block vision and movement in both directions.
• "objects_line_of_sight" data would result in rings of directional walls that block movement and block vision leaving the object.
• "portals" would be treated as windows. If the "closed" flag for a portal is true, the resulting wall would block both movement and vision. Otherwise, the wall will block movement but not vision.

Alternatives that you've considered.

We could force the use of walls instead of giving a preference. But given how new walls are, and the fact that some might want to be able to reproduce previous imports, I think leaving in a way to use masks is a good idea, at least for now.

Additional Context

No response