Add hit-detection support for WebGL

This PR implements `forEachFeatureAtPixel` for WebGL. From the idea it uses the same technique as the canvas renderer: We have a 1x1px [framebuffer](https://github.com/tsauerwein/ol3/blob/e01e556e3ab1a3ae4af2334d2bb23a84a22614f4/src/ol/webgl/context.js#L301) in which we [draw feature-by-feature](https://github.com/tsauerwein/ol3/blob/e01e556e3ab1a3ae4af2334d2bb23a84a22614f4/src/ol/render/webgl/webglreplay.js#L732) (in reverse order) and [check](https://github.com/tsauerwein/ol3/blob/e01e556e3ab1a3ae4af2334d2bb23a84a22614f4/src/ol/render/webgl/webglreplay.js#L1063) if there is some color.

Commit e01e556 uses an optimization, that should also be used for the canvas hit-detection: Using the new `renderBuffer` we build a box around the given position. And then we only check those features that intersect this box. This drastically improves the performance for WebGL.

Speaking about performance, canvas gives better results. I made some tests showing 1000, 5000, ... features on a map (zoomed-out so that really all features are shown). Then I measured the time to do a `forEachFeatureAtPixel` call.

    # of features   Canvas    WebGL
    1000            10 ms     20 - 80 ms
    5000            35 ms     70 - 350 ms
    10000           60 ms     140 - 650 ms
    20000           120 ms    340 - 1400 ms
    50000           320 ms    800 - 3000 ms 

For WebGL I have given a range, because it depends on where you click with the optimization made in e01e556. If you click somewhere in the map where are no features, it's faster.

But still, for WebGL, it's too slow to run `forEachFeatureAtPixel` on `mousemove`. We came up with a `hasFeatureAtPixel` method, for which I will open a separate PR ([here](http://tsauerwein.github.io/ol3/webgl-point-hasFeatureAtPixel/examples/icon-sprite-webgl.html) is already an example to give you an idea).
The above, texture-related, code should probably shared. It is not specific to hit detection. If we change the filtering parameters for the rendering to the screen I don't want to have to change some other code for the hit detection.