Blob Brush: Tool Options set the brush's Angle and Roundness

Blob Brush Tool Options gain Angle (-180 to 180 degrees, 0 by default)
and Roundness (0 to 100 percent, 100 by default), with a preview of the
tip and three dabs; a drag in the preview square sets the Angle. The
elliptical tip keeps its angle on the page along a stroke, a 0 percent
tip is drawn 0.1 percent as wide as it is long, and Size set to
Pressure still varies its size. path.blob takes angle and roundness,
and both values are kept with the tool's options.

While a non-round tip drags, the Blob Brush shows the outline of the
area it covers with its length label, and an Alt drag that smooths
shows the Smooth tool's thin line.

docs/control-protocol.md

@@ -814,7 +814,8 @@ Paintbrush, Blob Brush; on by default for the Pencil and Paintbrush and off for
 the path a stroke draws, continues or merges into is not selected afterwards), `mergeOnlySelection` (Blob Brush;
 off by default, and while it is on a stroke merges only with selected blobs), `closeWithin` and `editWithin`
 (screen pixels, 0 turns it off; Pencil, Paintbrush), `size` (pt; Blob Brush, Eraser), and `angle`
-(°, −180 to 180; 0) and `roundness` (%, 1 to 100; 100) of the Eraser's tip (Eraser). `ui.dialog.confirm` runs
+(°, −180 to 180; 0) and `roundness` (%, 0 to 100 for the Blob Brush, where 0 is drawn as 0.1, and 1 to 100 for
+the Eraser; 100) of the tip (Blob Brush, Eraser); the tip keeps its angle on the page along a stroke. `ui.dialog.confirm` runs
 `tool.setOption {tool, values}`.
 
 `ui.pointer` events also take `holdMs` (0..60000): the pointer then holds still that long, button down, before the