Fix cursor movement for large logos (#9628)

Fix cursor movement for large logos (#9628)

Previously, if one clicked onto a large non-editable inset like the new LyX
logo inset, the cursor was always positioned in front of the inset, even if
the click was almost at the back edge. Now the cursor is positioned at the
correct edge. I tested this also with RTL contents, where from means right
and back means left, but the inset anchor position anchor point is still
at the left, and the right edge is dim.wid pixels to the right of it.