gpu, io/router, op: use f32.Affine2D instead of op.TransformOp for transforms

Encode TransformOp as an Affince2D matrix instead and use that in gpu and io transform handling.
There are no changes to user facing API and so far only the offset part of the matrix is used.

This patch is a step towards full affine transformations.

Signed-off-by: Viktor <viktor.ogeman@gmail.com>

gpu/gpu.go

@@ -69,7 +69,7 @@ type drawOps struct {
 
 type drawState struct {
 	clip  f32.Rectangle
-	t     op.TransformOp
+	t     f32.Affine2D
 	cpath *pathOp
 	rect  bool
 	z     int
@@ -691,8 +691,8 @@ loop:
 		case opconst.TypeProfile:
 			d.profile = true
 		case opconst.TypeTransform:
-			dop := ops.DecodeTransformOp(encOp.Data)
-			state.t = state.t.Multiply(op.TransformOp(dop))
+			dop := ops.DecodeTransform(encOp.Data)
+			state.t = state.t.Mul(dop)
 		case opconst.TypeAux:
 			aux = encOp.Data[opconst.TypeAuxLen:]
 			auxKey = encOp.Key