Make get_output_next() work with non-aligned RandR setups (+test) (Thanks Feh, swh...

Make get_output_next() work with non-aligned RandR setups (+test) (Thanks Feh, swh, Moritz)

A good visualization of the new algorithm is this:

           +--------+
           |        |
+--------+=|   S1   |========================
|        | |        |
|   S0   | +--------+
|        |         +--------+
+--------+=========|        |================
                   |   S2   | +--------+
                   |        | |        |
                   +--------+ |   S3   |
                              |        |
                              +--------+

When focus is on S0, 'focus output right' will first match S1 (the
closest output which overlaps in the highlighted area), then S2, but not
S3 (since S3 does not overlap into the highlighted area).

fixes #669
fixes #771