Include multiple coverage reports in html with different coloring

[RobertBuhren]

Is your feature request related to a problem? Please describe.
I'm trying to identify code areas which are executed by INPUT_A but not by INPUT_B. I can generate two distinct coverage reports and then create two different html reports, but its not easy to see then where the difference in coverage is.

Describe the solution you'd like
Ideally I'd like to be able to point gcovr to multiple folders containing .gcda files generated by different inputs. In the html view I then can see different colored lines for each input/run. For example: Green - covered by both inputs, Red - not covered by any input, blue - only covered by INPUT_A, yellow - only covered by INPUT_B

Describe alternatives you've considered
So far I create two distinct reports and compare them manually

Additional context
N/A

Thanks!

Regards,

Robert

[Spacetown]

Such a compare can be done on the base of the JSON as described in #507 (comment). Creating a HTML report out of this is a quite big effort.