Fix offset in CanvasItem drag movement

[pablodp606]

The initial offset was using the mouse position when the threshold
was reached instead of the initial click position. This was adding
the drag threshold distance as an extra offset to the final
transformation.

Now the offset is calculated from the initial click position.

Before (notice the initial and the final mouse position over the sprite):

2021-11-26.01-00-41.mp4

After

2021-11-26.00-58-25.mp4

[Calinou]

For reference, the drag distance threshold was originally implemented in #49310.

The same change should most likely be performed in the 3D editor (editor/plugins/node_3d_editor_plugin.cpp), since the bug is probably present there too.

[JFonS]

The 3D equivalent would be to set

godot/editor/plugins/node_3d_editor_plugin.cpp

Line 1752 in b43281c

_compute_edit(_edit.mouse_pos);

to _compute_edit(_edit.original_mouse_pos);.

It's barely noticeable by default because the perspective changes and there's no reference point, but it does make a difference in orthogonal mode.

[pablodp606]

Ok, updated to include the fix for the 3D editor.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉