Draw Grid Mode: fixed a problem where "continuous drawing" would redraw all previously drawn shapes each time a drawing was finished. Fixes #912

UDBScript: fixed an issue where the Thing's class getSector method did not return null when the thing was not in a sector. Fixes #897

(Text labels are removed because of performance issues)

Added action to change the index of linedefs, things, sectors, and vertices. Can be accessed through a shortcut (not bound by default) or the menu of the respective mode (#903)

- Disable Visual Mode view stretching
- Disable increased texture brightness in classic modes
- Increased default Visual Mode FOV from 80 to 90