Merge pull request #1934 from fredj/ol.dom.createCanvasContext

Add ol.dom.createCanvasContext function
Set render order to null in ol.layer.Heatmap

For performance reasons: the render order is not relevant for a
heatmap representation and this avoids to sort the features each frame.
Merge pull request #1937 from fredj/heatmap-render-order

Set render order to null in ol.layer.Heatmap
Add method for testing whether the canvas size is valid

This is a way to detect whether a canvas exceeds the maximum
dimensions if these are limited on the target device. See
https://developer.apple.com/library/ios/documentation/AppleApplications/Reference/SafariWebContent/CreatingContentforSafarioniPhone/CreatingContentforSafarioniPhone.html
for limitations on iOS devices.
Allow to make canvas smaller when it is too big

For performance reason, we only make the canvas bigger, but not
smaller. With this change, we also make it smaller, but only
when we know that its current size exceeds the maximum
dimensions.
Change default renderer to Canvas
Merge pull request #887 from tschaub/default-renderer

Use the Canvas renderer by default.
Initialize canvasTooBig_ member
Make testCanvasSize a static function
Merge pull request #1928 from ahocevar/recover-from-canvas-too-big

Recover from situations where the maximum canvas size is exceeded
Merge pull request #1896 from ahocevar/nodragclick

Add click event as responsive alternative to singleclick
Handle skipped features when setting the map

When a Select interaction is removed from the map, it should
remove its selected features from the map's skippedFeatures
collection. When it is added to a map, it should add them.
Rename ol.source.VectorFile to ol.source.StaticVector
Factor out ol.source.FormatVector
Rename ol.source.FormatVector#loadFeatures to loadFeaturesFromURL
Load features before rendering
Add ol.loading
Add ol.source.RemoteVector
Load features on demand in vector-osm example
Export ol.format.OSMXML