go.*,text: implement shaper-driven line truncation

This commit pushes limiting the maximum number of lines of text into
the shaper implementation. This is more efficient than doing it in
widgets, and also opens the door for future use of the shaper to
insert ellipsis and other truncating characters as appropriate.

I realized that we lost the implementation of limiting the number of
lines of text in my text stack overhaul, so this fixes a regression
from that work.

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

text/shaper_test.go

@@ -10,6 +10,36 @@ import (
 	"golang.org/x/image/math/fixed"
 )
 
+// TestWrappingTruncation checks that the line wrapper's truncation features
+// behave as expected.
+func TestWrappingTruncation(t *testing.T) {
+	// Use a test string containing multiple newlines to ensure that they are shaped
+	// as separate paragraphs.
+	textInput := "Lorem ipsum dolor sit amet, consectetur adipiscing elit,\nsed do eiusmod tempor incididunt ut labore et\ndolore magna aliqua."
+	ltrFace, _ := opentype.Parse(goregular.TTF)
+	collection := []FontFace{{Face: ltrFace}}
+	cache := NewShaper(collection)
+	cache.LayoutString(Parameters{
+		Alignment: Middle,
+		PxPerEm:   fixed.I(10),
+	}, 200, 200, english, textInput)
+	untruncatedCount := len(cache.txt.lines)
+
+	for i := untruncatedCount + 1; i > 0; i-- {
+		cache.LayoutString(Parameters{
+			Alignment: Middle,
+			PxPerEm:   fixed.I(10),
+			MaxLines:  i,
+		}, 200, 200, english, textInput)
+		lineCount := len(cache.txt.lines)
+		if i <= untruncatedCount && lineCount != i {
+			t.Errorf("expected %d lines, got %d", i, lineCount)
+		} else if i > untruncatedCount && lineCount != untruncatedCount {
+			t.Errorf("expected %d lines, got %d", untruncatedCount, lineCount)
+		}
+	}
+}
+
 // TestCacheEmptyString ensures that shaping the empty string returns a
 // single synthetic glyph with ascent/descent info.
 func TestCacheEmptyString(t *testing.T) {