Support dynamic panelreel focus

[dankamongmen]

Right now, the focused tablet is always drawn at the top of the panelreel. The rules ought instead be:

• when focusing to a wholly on-screen tablet, do not move tablets, but only the focus
• when focusing to a tablet down, the focus ought be at the bottom of the displayed tablets
• when focusing to a tablet up, or receiving the initial focus, the focus ought be at the top

[dankamongmen]

Two problems immediately come up:

• when we're able to fit everything on the screen, moving focus down flips the first tablet down below the second one, leaving a gap. instead, everything ought be left where it is.
• otherwise, we hit an assert() in panelreel_draw_tablet() to catch the unhandled drawup case :)

[dankamongmen]

This is mostly working. Still needing resolution:

• #69 (nice) we're leaving focused tablets partially on-screen (bottom only). need to pre-enlarge.
• Need to make a decision on dealing with the case of an unfilled reel
  • Can tablets span the boundary? Ideally no.
  • Do we ever allow a gap of more than one line on the top? Not allowing such gaps can force the focused window to move. Allowing them looks weird and complicates validation.
• Seeing some weirdness at the top of the reel when focus is elsewhere, mainly blank lines or 2-line tablets with both top and bottom borders (and thus no content) done in #73
• Validation is currently disabled in panelreeldemo due to the topgap issue mention above

Ideally I can knock these out tonight, and this will be done!

[dankamongmen]

With regards to the "weirdness at the top of the reel": the most fundamental thing i'm seeing is that we're always off by one regarding the lines we print; we print up the penultimate one pretty much all the time when clipped at the top:

╭────────────────────────────────────────────────────────────────────────╮
│[#3 23 lines 0/16] 88888888888888888888888888888888888888888888888888888│
│999999999999999999999999999999999999999999999999999999999999999999999999│
│aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa│
│bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb│
│cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc│
│dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd│
│eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee│
│ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff│
│000000000000000000000000000000000000000000000000000000000000000000000000│
│111111111111111111111111111111111111111111111111111111111111111111111111│
│222222222222222222222222222222222222222222222222222222222222222222222222│
│333333333333333333333333333333333333333333333333333333333333333333333333│
│444444444444444444444444444444444444444444444444444444444444444444444444│
│555555555555555555555555555555555555555555555555555555555555555555555555│
│666666666666666666666666666666666666666666666666666666666666666666666666│
╰────────────────────────────────────────────────────────────────────────╯

here we have 23 (0x17) lines, and thus we ought be seeing 9..23 rather than 8..22. though really, of course, we ought be seeing 8..22, and that top border ought be clipped.

[dankamongmen]

Closing this, and making new bugs for the issues we're seeing.