fix: arrow tool glitches

[UnkwUsr]

Demo (on the left is fixed version, on the right is original flameshot):

demo_flameshot_bug_fix.mp4

[UnkwUsr]

Also check comment in code. The problem is that if we set length to 0, then "line" will be just 1 "pixel" (but real size is much bigger, because we set pen size before, in function ArrowTool::process (code block attached)), so we lose information about angle.

flameshot/src/tools/arrow/arrowtool.cpp

Line 148 in bcb4041

painter.setPen(QPen(color(), size()));

[borgmanJeremy]

Thanks!