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/font.go

@@ -39,6 +39,12 @@ func loadFont(b []byte) *font {
 	return &font{ascent: int(b[4]), descent: int(b[5]), n: n, index: b[10 : 10+8*n], data: b}
 }
 
+// Has reports whether the font draws r, not tofu.
+func Has(r rune) bool {
+	_, ok := font12.find(r)
+	return ok
+}
+
 func (f *font) find(r rune) (glyph, bool) {
 	lo, hi := 0, f.n
 	for lo < hi {