Client doesn't show currently visible area on the minimap correctly in some cases

[daavko]

Describe the bug When playing on a non-iso map with an iso tileset (or the other way around), the client doesn't show all borders of currently visible area on the minimap - one of the lines always disappears before going off-screen (or rather, off-minimap). This only happens when viewing an area in a corner of the minimap.

To Reproduce Steps to reproduce the behavior:

1. Start a game on a non-iso map with an iso tileset
2. Move the map view to an area in one of the corners of the minimap
3. Observe one of the lines depicting the current area missing

Expected behavior I expect all lines to be there

Screenshots What happens What I expect (missing line in magenta)

Platform and version (please complete the following information):

• OS: Windows 10
• Freeciv21 version: beta 3
• Ruleset/Longturn game (if applicable): not applicable

Additional context No additional context