Use window-relative anchor position when moving windows

From the main commit:

The anchor position calculations are somewhat unnecessarily complex
based on root coordinates of pointer and frame positions. This requires
tracking both things, and we don't always get it quite right with the
latter (e.g. window repositions, resizes or overshrinks, leaving the
anchor position visually outside the window).

In order to improve this, capture the window-relative coordinates
when starting the window drag, and ensure the window is always repositioned
in that position, relative to its current size.

This avoids these glitches when unmaximizing a window (e.g. dragged from
the bottom through super+button1 press), or moving windows between monitors
with different scales.

Closes: #2730 (closed)