clear method should remove references to features

[tschaub]

With the elements renderers, you can call clear to effectively erase features from the renderer. With the canvas renderer, those features come back on the next draw.

[tschaub]

remove feature map

[tschaub]

An alternative if we don't like the duplicated line of code would be to add a "soft" arg to clear. Called with soft true, feature references would not be removed.

As is, tests pass (and clear permanently clears).

[crschmidt]

This looks fine to me. If the general meaning of clear() is 'get rid of and don't draw again until I tell you otherwise", the Canvas renderer is currently breaking that; fixing that makes sense. Code looks solid, and doesn't appear to break anything.

[tschaub]

(In [10595]) Making it so the clear method on the canvas renderer permanently clears features (as with the other renderers). r=crschmidt (closes #2775)