Size the grid by caps, not by tile size

tile-width and tile-height are gone. In their place the config states a
box and a grid, all four settings caps of the same kind:

    max-width   = 90ppt      # the box the grid may fill
    max-height  = 90ppt
    max-columns = 4          # the grid inside it
    max-rows    = 4

A thumbnail is that box divided by those caps. The property that buys is
that a thumbnail's size no longer depends on how many windows are open:
one window gets the same thumbnail as thirty, in a smaller overlay,
because the overlay hugs whatever is actually there. Rows past max-rows
still scroll.

It also removes two things that were hard to explain. tile-height used to
default to the display's aspect through a rule you could only learn from
the documentation — the shape now falls out of the box and the grid.
And ppt meant "of the display width" on one key and "of the display
height" on another; the two remaining lengths take the axis their name
implies.

The 90% fill constant went with them: it was an invisible cap doing the
job max-width now does out loud, and its old value is the default.

Turning labels off now gives that row to the thumbnails rather than
shrinking the window, which follows from the box being what you asked
for. Verified on a 1280x1440 display: 90ppt gives a 1149x1293 overlay,
60ppt gives 765x861, and caps of 2x2 in the same box give larger
thumbnails that scroll after two rows.

README.md

@@ -150,10 +150,10 @@ selection-text = #282828      # its label
 border         = #d79921
 border-width   = 2px
 
-tile-width     = 18ppt        # largest a thumbnail may be
-tile-height    = 20ppt        # defaults to the display's aspect
-max-columns    = 4
-max-rows       = 3            # default: however many the display fits
+max-width      = 90ppt        # the box the grid may fill
+max-height     = 90ppt
+max-columns    = 4            # thumbnails are that box divided by these
+max-rows       = 4
 
 font           = monospace
 font-size      = 13.3
@@ -164,30 +164,27 @@ fps            = 12
 format         = tsv
 ```
 
-Sizes take sway's units: `600px` is absolute, `70ppt` a percentage — and the
+Sizes take sway's units: `600px` is absolute, `90ppt` a percentage — and the
 percentage resolves against **the display the grid actually appears on**, every
-time it runs. On a mixed setup one file gives 18% of a 1280-wide laptop panel and
-18% of a 3840-wide monitor, instead of a pixel count that suits one and looks
-wrong on the other. The overlay is mapped explicitly on that display, at that
-display's scale, so mixed-DPI renders crisply either way.
+time it runs. On a mixed setup one file gives 90% of a 1280-wide laptop panel and
+90% of a 3840-wide monitor, rather than a pixel count that suits one and looks
+wrong on the other. The overlay maps explicitly on that display, at its scale, so
+mixed-DPI renders crisply either way.
 
-`tile-width` and `tile-height` set how big a thumbnail actually is. Give only
-the width and the height follows the display's aspect, which is roughly the shape
-of the windows on it — a 16:9 cell wastes about half its area on a portrait
-monitor.
+All four sizing settings are **caps**:
 
-Nothing sets the overlay's height directly: it is as many rows as fit in 90% of
-the display, so below that threshold the window hugs the grid. `max-rows` caps it
-if you would rather have a compact strip that scrolls sooner than a full-height
-overlay — the symmetric partner to `max-columns`.
+- `max-width` and `max-height` bound the overlay.
+- `max-columns` and `max-rows` bound the grid inside it.
 
-When there are more rows than can be shown, **the grid scrolls**: the tile size
-you asked for is honoured and a scrollbar appears in the right margin.
-Any move keeps the selection in view, `PgUp`/`PgDn` jump a screen, and tiles
-scrolled out of sight are unmapped — so live capture skips them too, which is
-what stops a long list costing bandwidth for pixels nobody sees. Only a tile too
-large for even one row or column is shrunk, since then nothing could be shown at
-all.
+A thumbnail is simply that box divided by those caps, which means **its size
+never depends on how many windows are open**: one window gets the same
+thumbnail as thirty, in a smaller overlay, because the overlay hugs whatever is
+actually there. Rows past `max-rows` scroll, with a scrollbar in the right
+margin, `PgUp`/`PgDn`, and the selection always kept in view. Tiles scrolled out
+of sight are unmapped, so live capture skips them too.
+
+Turning labels off gives that row back to the thumbnails rather than shrinking
+the window, since the box is what you asked for either way.
 
 ## Look