op/paint: remove support for PaintOp.Rect

PaintOp.Rect is the wrong abstraction; it implies a clip operation
better handled by package clip, and not all paints need it (colors).
Furthermore, it's awkward to specify a PaintOp that fills up the
current clip area, regardless of its size.

Redefine PathOp to mean "fill current clip area".

API change. Replace uses of PaintOp.Rect with a TransformOp applied
before the PaintOp.

Leave a TODO for the PathOp infinity area.

Fixes gio#167

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/rendertest/clip_test.go

@@ -81,7 +81,8 @@ func TestPaintArc(t *testing.T) {
 func TestPaintTexture(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		squares.Add(o)
-		paint.PaintOp{Rect: f32.Rect(0, 0, 80, 80)}.Add(o)
+		scale(80.0/512, 80.0/512).Add(o)
+		paint.PaintOp{}.Add(o)
 	}, func(r result) {
 		r.expect(0, 0, colornames.Blue)
 		r.expect(79, 10, colornames.Green)
@@ -94,7 +95,8 @@ func TestPaintClippedTexture(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		squares.Add(o)
 		clip.RRect{Rect: f32.Rect(0, 0, 40, 40)}.Add(o)
-		paint.PaintOp{Rect: f32.Rect(0, 0, 80, 80)}.Add(o)
+		scale(80.0/512, 80.0/512).Add(o)
+		paint.PaintOp{}.Add(o)
 	}, func(r result) {
 		r.expect(40, 40, colornames.White)
 		r.expect(25, 35, colornames.Blue)