Calculate row bounding box in single-word mode per #4304

[Balearica]

See #4304 for full context.

In short, ROW objects are expected to have valid/accurate bounding boxes, which are accessed using row->bounding_box(). However, bounding box values are not calculated automatically--the method row_obj->recalc_bounding_box() must be run after the row is created and populated to calculate the bounding box. This happens correctly in 5 of the 6 instances where new ROW is called, however does not occur in 1 case (within the make_single_word function that runs when psm is SINGLE_WORD or SINGLE_CHAR).

This PR fixes this by adding the required call to recalc_bounding_box within make_single_word. This brings that function in line with all the other functions that create ROW objects.

This change fixes the bug described in #4304, where baselines are not correctly calculated when using certain psm settings. After implementing this fix, the motivating example in that issue is resolved, and the baseline is calculated correctly.

[stweil]

@Balearica, thanks for this fix. Please use your full name as commit author in future pull requests.