This uses a similar approach to that used by tmux; when resizing,
a size delta is computed in each axis and that value is distributed
across the splits in that dimension.
For a small resize by 1 cell this tends to bias towards adjusting
the top/left node.
For a large resize (eg: maximizing/restoring) the distribution tends
to share across the splits in that dimension, effectively scaling
them approximately proportionally, more or less preserving the relative
scale of the splits.
refs: https://github.com/wez/wezterm/issues/157#issuecomment-699672914
This commit provides the heart of the split functionality.
It adds a couple of key assignments (that are not finalized)
for splitting the active pane horizontally or vertically.
Clicking in a pane will activate it.
Selection is constrained to the active pane.
Resizing a window will grow/shrink the right/bottom side of a split.
The scrollbar applies to the active split.
There's not yet a way to resize a split at all.
There's not yet a key assignment for changing the active split.
The software renderer doesn't know how to render the splits correctly.
refs: https://github.com/wez/wezterm/issues/157
