A `backquote` given as a string or a number, on the request or on an
event, was read as absent, so `"true"` moved the object's geometry
instead of its pattern. `pointer_gesture` and `ui.pointer`, headless
and desktop, now answer with an error before any event runs.
Holding backquote while dragging with the Selection tool (move, bounding-box
scale or rotate) or the Rotate, Reflect, Scale or Shear tool applies the drag
to the selected objects' pattern fills and strokes and leaves their geometry
in place. The key state at release decides the result: the key may be pressed
after the mouse button goes down, and releasing it before the button comes up
commits an ordinary transform. Objects without pattern paint do not move.
PointerEvent gains a `backquote` flag (false when absent). The desktop canvas
reads it from the held key on every pointer event, and `ui.pointer` and the
`pointer_gesture` tool accept it per event or as a default for the gesture.
dotCooCoo