op/paint: add support for efficient ImageOp subimages

The new field ImageOp.Rect is initialized to cover the entire source
image, but can be modified to draw only a section of it.

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

internal/opconst/ops.go

@@ -35,7 +35,7 @@ const (
 	TypeTransformLen    = 1 + 4*2
 	TypeLayerLen        = 1
 	TypeRedrawLen       = 1 + 8
-	TypeImageLen        = 1
+	TypeImageLen        = 1 + 4*4
 	TypePaintLen        = 1 + 4*4
 	TypeColorLen        = 1 + 4
 	TypeAreaLen         = 1 + 1 + 4*4