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
Now values which are not mentioned in the style are still added to
the string literals mapping.
Also an error will be thrown if a style references a missing variable.
Type checking is done either against a literal value (number, string...)
or against the operator in case of an expression.
Sometimes it is not possible to infer only one type, for example
the value 'transparent' could either be a color or a string. This
is covered by the fact that all operators expect exactly one type
for their arguments.
These variables are accessed using the `var` operator, and are meant
to be mutated directly in the style object. When doing so,
the new value will always be used for the render.
This is intended to be used for modifying the style through external
parameters such as an input field, a slider, mouse position etc.
There is now a ShaderBuilder class with chainable methods
to specify more easily the contents of the final shaders.
This is to avoid passing around large objects to different functions,
and allow for a richer API.
The documentation has also been corrected and clarified.
Now most attributes will be custom ones defined by the user of the renderer.
The hit detection is broken for now and still has to be fixed.
Also it is not possible anymore to give a texture to the renderer,
this will have to be fixed as well.
Now two methods are available: `readAll` and `readPixel`,
and the data from the render target is not re-read every time unless
`clearCachedData` is called.
