widget{,/material}: rebuild label and editor with textView

This commit rebuilds the editor and label types on the common
foundation provided by textView. This enables labels to have
optional state that makes them selectable, and allows the
two widgets to share the code for managing cursor positions,
displaying selections, and soforth. Labels now have an additional
Layout function which can be invoked if they have a Selectable.
It accepts a layout.Widget used to paint their contents. Stateless
labels should still use the old Layout method.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/text.go

@@ -37,49 +37,6 @@ type textSource interface {
 	ReplaceRunes(byteOffset int64, runeCount int64, replacement string)
 }
 
-type maskReader2 struct {
-	// rr is the underlying reader.
-	rr      io.RuneReader
-	maskBuf [utf8.UTFMax]byte
-	// mask is the utf-8 encoded mask rune.
-	mask []byte
-	// overflow contains excess mask bytes left over after the last Read call.
-	overflow []byte
-}
-
-func (m *maskReader2) Reset(r io.Reader, mr rune) {
-	m.rr = bufio.NewReader(r)
-	n := utf8.EncodeRune(m.maskBuf[:], mr)
-	m.mask = m.maskBuf[:n]
-}
-
-// Read reads from the underlying reader and replaces every
-// rune with the mask rune.
-func (m *maskReader2) Read(b []byte) (n int, err error) {
-	for len(b) > 0 {
-		var replacement []byte
-		if len(m.overflow) > 0 {
-			replacement = m.overflow
-		} else {
-			var r rune
-			r, _, err = m.rr.ReadRune()
-			if err != nil {
-				break
-			}
-			if r == '\n' {
-				replacement = []byte{'\n'}
-			} else {
-				replacement = m.mask
-			}
-		}
-		nn := copy(b, replacement)
-		m.overflow = replacement[nn:]
-		n += nn
-		b = b[nn:]
-	}
-	return n, err
-}
-
 // textView provides efficient shaping and indexing of interactive text. When provided
 // with a TextSource, textView will shape and cache the runes within that source.
 // It provides methods for configuring a viewport onto the shaped text which can
@@ -102,7 +59,7 @@ type textView struct {
 	textSize           fixed.Int26_6
 	seekCursor         int64
 	rr                 textSource
-	maskReader         maskReader2
+	maskReader         maskReader
 	lastMask           rune
 	maxWidth, minWidth int
 	viewSize           image.Point