This is more work to further isolate the layers that entities draw to.
It makes it easier to debug what is going on, and can eventually lead to
deferred drawing, if each draw function is in its own place and not dependant
on anything else.
I've started to replace the vertex-hover with an explicit layer for touch
targets.
Also had to change a lot of the svg tests, which are really brittle.
Things would happen like - the surface would be created, it would kick of a
deferred redraw, which would notice that the zoom was 0 and call
editOff, which would remove the osm layers that were just created and
that the tests were trying to draw to. These tests need proper zoom and
projection otherwise nothing works.
(re: #3003 / #4602)
For now, drawHover is commented out. Still not sure what I will do with it.
This means that things flicker a bit when dragging, also connecting nodes
(and closing lines) does not currently work.
There was lot going on preventing the vertices from rendering while dragging.
1. `modeDragNode` needed a proper `selectedIDs()` function that works like other
modes.. Many other places in iD (including the vertex renderer) call `context.selectedIDs()`..
This means that `modeDragNode` needs a new function `restoreSelectedIDs()` to do what
`selectedIDs()` was previously doing (a place to store selectedIDs so that we
can reselect those entities after the user is done dragging a node in select mode)
2. Just so many things in svg/vertices.js
- siblingAndChildVertices was missing some things for points that we render
as vertices (points in wireframe, points with directions)
- the sibling vertices weren't being included in the `filter` function, so
would disappear when doing differenced/extent redraws
- probably some other things
