texthandle: Set a bigger input shape, covering the line height

texthandle: Set a bigger input shape, covering the line height

Now, even if the handles being rendered are small, the handle touch
input shape will be as wide as the visible part of the rendered asset, and
high enough to cover both the handle and the height of the line where
the selection bound is.

Also, make handles have the same virtual distance to the line top/bottom
when a drag starts, so the handle doesn't jump to another line after a
too short threshold.