Nested frames have longer horizontal lines that match their parent

[Zeta611]

Nested frames have longer horizontal lines that match their parent:

utop # PrintBox.(
      frame
        (vlist
           [
             text "123456789";
             frame (vlist [ text "." |> align ~h:`Right ~v:`Top; text "." ]);
           ]));;
- : PrintBox.t =
┌─────────┐
│123456789│
├─────────┤
│┌─┐      │
││ │    . │
│├─┼───── │
││.│      │
│└─┘      │
└─────────┘

This breaks alignment as shown above.

This even causes an assertion failure:

utop # PrintBox.(
      frame
        (vlist
           [
             text "123456789";
             frame (vlist [ text "...." |> align ~h:`Right ~v:`Top; text "." ]);
           ]));;
- : PrintBox.t =
┌─────────┐
│123456789│
├─────────┤
│┌────┐   │
││   ....<printer PrintBox_text.pp raised an exception: File "src/printbox-text/PrintBox_text.ml", line 202, characters 6-12: Assertion failed>

when the right aligned text intersects with the frame which "(logically) contains" it

[c-cube]

Should be fixed in #46 . I also added a stretch parameter to frame, see the example test (to pick whether you want the frame to expand or not).

[Zeta611]

Thanks for the super fast fix!