Trying to fine-tune position of one end of line in Control Points grabs the wrong end of the line

[Jakub Narębski]

When trying to straighten the image, both panes in the "Control Points" tab in main Hugin window, one needs to provide horizontal and vertical lines. To do that, one needs to select the beginning point of line in one pane, and the end point of line in the other pane of "Control Points" tab. Usually Hugin fine-tunes the endpoints correctly, but sometimes one needs to fine-tune the position (e.g. if Hugin moved point to parallel line with more contrast).

But when I try to grab endpoint that I have created in right pane, Hugin catches the other endpoint instead. I have to switch panes, that is use left pane for end of line, right pane for beginning of the line.

Steps:
------
1. Switch to "Control Points" tab
2. Ensure that both panes display the same image; zoom is set to "fit to window", all three checkboxes (auto fine-tune, auto add, auto estimate) are checked - this is the default setting
3. Select left endpoint of horizontal line on left pane (start of line)
4. Select right endpoint of horizontal line on right pane (end of line)
5. Switch to 100% zoom or more for fine-tuning; usually whole image does not fit into pane
6. Try to grab right endpoint on right pane (the one that was just added)

Expected:
---------
1. You grab visible endpoint under cursor, and are able to fine-tune its position

Actual result:
--------------
1. You grab the endpoint that was last manipulated previously (was added in the left pane, and/or manipulated there).
2. If you hold mouse button longer, it would move starting endpoint to unexpected position

Workaround:
-----------
1. Switch panes: use left pane to fine-tune end of line, and right pane to fine-tune beginning of line.

System and version:
-------------------
Operating System: Windows 10 (build 16299), 64-bit edition
Architecture: 64 bit
Free memory: 1685604 kiB
Active Codepage: 1250 (Central European Windows)

Hugin
Version: 2018.0.0.5abfb4de7961 built by Thomas
Path to resources: C:\Program Files\Hugin\share\hugin\xrc\
Path to data: C:\Program Files\Hugin\share\hugin\data\
Hugins camera and lens database: C:\Users\jnare\AppData\Roaming\hugin\camlens.db
Multi-threading using C++11 std::thread and OpenMP
Monitor profile: C:\WINDOWS\system32\spool\drivers\color\sRGB Color Space Profile.icm

Libraries
wxWidgets: wxWidgets 3.1.1
wxWidgets Library (wxMSW port)
Version 3.1.1 (Unicode: wchar_t, debug level: 1),
compiled at Nov 18 2017 10:28:15

Runtime version of toolkit used is 10.0.

libpano13: 2.9.19
Exiv2: 0.26
SQLite3: 3.18.0
Vigra: 1.11.0
LittleCMS2: 2.9

[tmodes (tmodes)]

The fine-tune function does only work with normal control points (because it is using auto-correlation).
I disabled therefore the fine-tune button for line control points.

[tmodes (tmodes)]

I reverted the changes concerning disable of fine-tune. Instead I modified the fine-tune code to better cope with line control points.