Dragging the cursor on ground or liquid while in drag mode for highlighting messes up packet positions

[AdamPlenty]

To reproduce, hold shift and the left mouse button on ground or liquid, and move the cursor to a diggable wall. Often, you'll get a box that extends beyond where you dragged the cursor while holding the left mouse button.

I have no idea what the heck is going on, but it's blocking #4187, as it causes tiles (sometimes from several slabs away!) to be tagged for digging when you try to pick things up.

[AdamPlenty]

This may have been fixed in 55d6361aabcd5402a435a90fc3988c3c12b8a0b6.