Turns out there are sometimes errors that will not prevent the
shader from functioning normally. As such, it is not possible
to simply throw when there are errors/warnings.
Since we are not logging to the console, these errors will have to stay hidden
unless the user requests them explicitly.
The methods `View.adjustRotation`, `View.adjustZoom` and `View.adjustResolution`
optionally take an `opt_anchor` parameter. When `opt_anchor` is `undefined`, the
methods work properly. When it is `null`, they currently attempt to access the
`null` value as though it is a `Coordinate` object, and throw:
```
TypeError: Cannot read property '0' of null
```
This change allows `opt_anchor` to safely be `null`.
Existing code paths already expect this to be supported. For example, when
`MouseWheelZoom.useAnchor_` is `false`, then
[`MouseWheelZoom.lastAnchor_` remains `null`](77bc6897dd/src/ol/interaction/MouseWheelZoom.js (L166));
thus, this `null` value
[is passed to `View.adjustZoom` as `opt_anchor`](77bc6897dd/src/ol/interaction/MouseWheelZoom.js (L209)).
Because not all elements in the DOM are instance of `HTMLElement`.
For example, SVGs created with `document.createElementNS` are instances of `Element`.
The hit detection render is now done against a canvas with half the
width/height of the main render. This still provides sufficient precision
while requiring a much smaller memory allocation (especially for
retina devices).
This would happen when WebGLPointsLayerRenderer.forEachFeatureAtCoordinate
is called on "warped" worlds at -360/+360 degrees, and may produce false
positives.
Various references were kept, preventing the layer and underlying
renderer and webgl context to be garbage collected.
Also, the Helper was simplified because it turns out deleting manually
all Webgl objects is useless: these objects will be released when
the context is garbage collected anyway.
Note: this touches the Layer and BaseLayer classes, as the following were
preventing the layer from being garbage collected:
* layer reference in the `state_` object in BaseLayer
* dangling listener for source change in Layer
