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>

op/paint/paint.go

@@ -16,6 +16,7 @@ import (
 
 // ImageOp sets the material to an image.
 type ImageOp struct {
+	Rect    image.Rectangle
 	uniform bool
 	color   color.RGBA
 	src     *image.RGBA
@@ -48,6 +49,7 @@ func NewImageOp(src image.Image) ImageOp {
 		bounds := src.Bounds()
 		if bounds.Min == (image.Point{}) && src.Stride == bounds.Dx()*4 {
 			return ImageOp{
+				Rect:   src.Bounds(),
 				src:    src,
 				handle: new(int),
 			}
@@ -82,6 +84,11 @@ func (i ImageOp) Add(o *op.Ops) {
 	}
 	data := o.Write(opconst.TypeImageLen, i.src, i.handle)
 	data[0] = byte(opconst.TypeImage)
+	bo := binary.LittleEndian
+	bo.PutUint32(data[1:], uint32(i.Rect.Min.X))
+	bo.PutUint32(data[5:], uint32(i.Rect.Min.Y))
+	bo.PutUint32(data[9:], uint32(i.Rect.Max.X))
+	bo.PutUint32(data[13:], uint32(i.Rect.Max.Y))
 }
 
 func (c ColorOp) Add(o *op.Ops) {