Add code components 366-385; ScrollArea Follow, editor sync

A terminal with history, a Go-colored code block, a log viewer that
follows, a hex viewer, a memory map, a JSON tree, a line diff, a GAME
OVER crash screen, an FPS meter, profiler bars, breakpoints,
variables, command history, a shortcut list, a debug grid, a pixel
editor that exports string art, a palette editor, a sprite sheet, a
glyph map and a 6502 register view.

layout.ScrollArea takes Follow, and ChatLog now uses it. data.TreeNode
takes a Value drawn in its Slot. pixel gains Has, and Sprite.Art
writes MustSprite art. pixel.Editor takes the caller's value at each
input event, so a value changed between frames is never typed onto.

pixel/editor.go

@@ -81,6 +81,10 @@ func Editor(c *ui.Context, value *string, o EditorOptions) Edit {
 	composing := cb > ca
 	base.TextCaret(ui.Rect{X: k.D(cx - scroll), Y: k.D(cl * LineHeight), W: k.dot, H: k.D(LineHeight)})
 	base.HandleInput(func(ev ui.InputEvent) bool {
+		// The caller may have changed the value since this frame.
+		if string(s.text) != *value {
+			s.reset(*value)
+		}
 		took := s.handle(ev, env)
 		if s.changed {
 			*value = string(s.text)