gpu: [compute] add function for separating integer offsets from transforms

Refactor only; separateTransform is needed in the following change.

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

gpu/compute.go

@@ -1679,18 +1679,14 @@ func encodeOp(viewport image.Point, absOff f32.Point, enc *encoder, texOps *[]te
 	case materialTexture:
 		// Add fill command. Its offset is resolved and filled in renderMaterials.
 		idx := enc.fillImage(0)
-		sx, hx, ox, hy, sy, oy := op.state.t.Elems()
-		ox += absOff.X
-		oy += absOff.Y
 		// Separate integer offset from transformation. TextureOps that have identical transforms
 		// except for their integer offsets can share a transformed image.
-		intx, fracx := math.Modf(float64(ox))
-		inty, fracy := math.Modf(float64(oy))
-		t := f32.NewAffine2D(sx, hx, float32(fracx), hy, sy, float32(fracy))
+		t := op.state.t.Offset(absOff)
+		t, off := separateTransform(t)
 		*texOps = append(*texOps, textureOp{
 			sceneIdx: idx,
 			img:      op.state.image,
-			off:      image.Pt(int(intx), int(inty)),
+			off:      off,
 			key: textureKey{
 				transform: t,
 				handle:    op.state.image.handle,
@@ -1729,6 +1725,14 @@ func transformBounds(t f32.Affine2D, bounds f32.Rectangle) rectangle {
 	}
 }
 
+func separateTransform(t f32.Affine2D) (f32.Affine2D, image.Point) {
+	sx, hx, ox, hy, sy, oy := t.Elems()
+	intx, fracx := math.Modf(float64(ox))
+	inty, fracy := math.Modf(float64(oy))
+	t = f32.NewAffine2D(sx, hx, float32(fracx), hy, sy, float32(fracy))
+	return t, image.Pt(int(intx), int(inty))
+}
+
 func transformRect(t f32.Affine2D, r rectangle) rectangle {
 	var tr rectangle
 	for i, c := range r {