Drag and drop header, more problems

[sanjayssk]

I found some new bugs related to drag and drop of header.

1. Drag a header entirely across the header area and don't move the mouse after a drop. The repaint does not occur and you might see old column name till you move the mouse.

2. Drag a header from column 3 to column 1 beginning (entirely over the header). The left drop mark appears on column 1 but when you drop, the actual drop does not occur. If you do the same operation over the columns area, it works.

Commented my earlier fixes in #248 and #643 just to see if they caused any of these problems. But the problems still occur.

[sanjayssk]

Problem 1 is a timing issue where the BitBlt of the drag object is slower than the painting of the changed header that occurs after it with the result that what you see after the drop is the backup image instead of the changed columns. When you move the mouse away from the header, it's repainted again. This problem seems to occur only on a certain graphics adapter on Windows 10. Will fix this if the fix seems simple enough so as not to disturb anything else.

Problem 2 is a case of detection of the drag as a drag out and so column change does not occur. But the fact that the drop mark appears and is not honored is not a correct behavior. Will need to investigate some more to see if it is worth solving.