Add test-suite using SlimerJS

test_rendering/README.md

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+# Rendering tests
+
+This directory contains rendering tests which compare a rendered map with a
+reference image using [resemble.js](http://huddle.github.io/Resemble.js/).
+
+Similar to the unit tests, there are two ways to run the tests: directly in the
+browser or using [SlimerJS](http://slimerjs.org/) from the command-line.
+
+To run the tests in the browser, make sure the development server is running
+(`./build.py serve`) and open the URL
+[http://localhost:3000/test_rendering/index.html](http://localhost:3000/test_rendering/index.html).
+
+From the command-line the tests can be run with the build target `./build.py test-rendering`.
+
+## Adding new tests
+When creating a new test case, a reference image has to be created. By appending `?generate`
+to the URL, a canvas with the rendered map will be shown on the page. Then the reference
+image can simply be created with a right-click and "Save image as".
+
+It is recommended to only run a single test case when generating the reference image.
+
+## Image difference
+When a test fails, an image showing the difference between the reference image and the
+rendered map can be displayed by appending `?showdiff` to the URL.