Combine coverage reports from multiple platforms

We may gather code coverage for several platforms, but we don't present the results sensibly in a single report.

The difficulty of this task highly depends on which coverage tools we use, which is not set in stone. If we manage to use the same coverage tool on all platforms then we should be able to merge results from different platforms much easier.

Various ways it might make sense to combine the results from different platforms:

• union - was this source tested on any platform?
• intersection - was this source tested on all platforms?
• just display all the data for all platforms and make the viewer figure it out