Building grid UI could skip open yard tiles

Consider this 5x5 yardmap:
```
yyoyy
yoooy
ooooo
yoooy
yyoyy
```
Right now placement GUI looks like this:

Which is slightly confusing since the corners overlap. The request is to give the option to only render the "real" tiles:

Remarks:
* this should be an option, the default should remain to render the full rectangle as-is
* i'm not sure if there are other fully ignored yardmap tiles like `y`, check and handle them as well
* i'm not specifying how to enable the option (springsetting? modrule? some lua callin/callout?), your choice

The issue names no files or tests. Start by locating the building placement GUI and yardmap handling, then determine how fully ignored tiles such as `y` are identified and how an opt-in setting can control rendering while preserving the current full-rectangle default; done means the example grid renders both modes correctly.

Written by the indexing model from the issue text.