deps,text,widget,font/opentype: [API] add harfbuzz-powered text shaper

This commit introduces a new text shaping infrastructure
powered by Benoit Kugler's Go source-port of harfbuzz.
This shaper can properly display complex scripts and RTL
text. This commit changes the signature of the text.Shaper
function, which is a breaking API change.

The new functionality is available via opentype.ParseHarfbuzz,
which configures a text.Shaper leveraging the new backend.

References: https://todo.sr.ht/~eliasnaur/gio/146
Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/label.go

@@ -58,7 +58,7 @@ func clipLine(lines []text.Line, alignment text.Alignment, width int, clip image
 
 func subLayout(line text.Line, startCol, endCol int) text.Layout {
 	adv := line.Layout.Advances
-	if startCol == len(adv) {
+	if startCol == line.Layout.Runes.Count {
 		return text.Layout{}
 	}
 	adv = adv[startCol:endCol]
@@ -90,7 +90,7 @@ func (p1 screenPos) Less(p2 screenPos) bool {
 func (l Label) Layout(gtx layout.Context, s text.Shaper, font text.Font, size unit.Value, txt string) layout.Dimensions {
 	cs := gtx.Constraints
 	textSize := fixed.I(gtx.Px(size))
-	lines := s.LayoutString(font, textSize, cs.Max.X, txt)
+	lines := s.LayoutString(font, textSize, cs.Max.X, gtx.Locale, txt)
 	if max := l.MaxLines; max > 0 && len(lines) > max {
 		lines = lines[:max]
 	}