The hover's context now includes the active tool (PR review follow-up):
switching to the Selection tool with Alt held and the pointer still
showed no Distance Guides until the pointer moved, since the new tool
had never hovered. A tool switch re-hovers at the pointer whatever the
tool; the rest of the context (canvas, view, document and revision)
re-hovers tools that ask (Tool::wants_rehover).
canvas::tests::distance_guides_follow_what_lies_under_a_still_pointer
adds a tool switch (fails without the rule), a modal dialog with its
backdrop over the pointer, and another document opened under it.
Taha-Mahmoodi
b321a77d38