coordsAtPos is bad at bidirectional text

[marijnh]

It makes assumptions about which side of a rectangle it should return that only hold in left-to-right text.

With patch e2cd163108 it is slightly better on Chrome and Firefox, but that approach (using an empty text range to query coordinates) makes it very hard to solve #899, so we'll probably need to find another solution.

I have no idea what that would look like—in simple cases you could use the position of nearby characters to figure out the direction of the text, but given a complex enough mix of left-to-right and right-to-left text such an approach tends to break down.