op/clip: expose LineTo and QuadTo

Using delta position with Line and Quad can drift over successive calls.
Also, in some cases it's much more convenient to use absolute
coordinates rather than relative.

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

internal/rendertest/clip_test.go

@@ -78,6 +78,26 @@ func TestPaintArc(t *testing.T) {
 	})
 }
 
+func TestPaintAbsolute(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		p := new(clip.Path)
+		p.Begin(o)
+		p.Move(f32.Pt(100, 100)) // offset the initial pen position to test "MoveTo"
+
+		p.MoveTo(f32.Pt(20, 20))
+		p.LineTo(f32.Pt(80, 20))
+		p.QuadTo(f32.Pt(80, 80), f32.Pt(20, 80))
+		p.Outline().Add(o)
+
+		paint.FillShape(o, red, clip.Rect(image.Rect(0, 0, 128, 128)).Op())
+	}, func(r result) {
+		r.expect(0, 0, colornames.White)
+		r.expect(30, 30, colornames.Red)
+		r.expect(79, 79, colornames.White)
+		r.expect(90, 90, colornames.White)
+	})
+}
+
 func TestPaintTexture(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		squares.Add(o)