Bad layout around “floating” image in manual 0.10.2

[OlivierNicole]

Page 4 (as numbered by SILE), the image breaks spacing somehow.

[alerque]

This is largely a duplicate of #394, something we've long known is a problem and still haven't worked out the math for.

Incidentally this same float in the manual also features in #560, although the issue there has to do with recalculating things after the float rather than calculating the baseline at the start of the float.

In any case I'm fine with keeping this issue open to track this specifically on the manual, but the fix will run deeper that just tweaking the manual ;-)