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/opconst/ops.go

@@ -36,7 +36,7 @@ const (
 	TypeLayerLen          = 1
 	TypeRedrawLen         = 1 + 8
 	TypeImageLen          = 1
-	TypePaintLen          = 1 + 4*4
+	TypePaintLen          = 1
 	TypeColorLen          = 1 + 4
 	TypeLinearGradientLen = 1 + 8*2 + 4*2
 	TypeAreaLen           = 1 + 1 + 4*4