all: clean up code, upgrade to modern Go

Signed-off-by: ddkwork

op/clip/clip.go

@@ -275,7 +275,7 @@ func (p *Path) QuadTo(ctrl, to f32.Point) {
 // negative clockwise.
 func (p *Path) ArcTo(f1, f2 f32.Point, angle float32) {
 	m, segments := stroke.ArcTransform(p.pen, f1, f2, angle)
-	for i := 0; i < segments; i++ {
+	for range segments {
 		p0 := p.pen
 		p1 := m.Transform(p0)
 		p2 := m.Transform(p1)

op/paint/paint.go

@@ -35,7 +35,7 @@ type ImageOp struct {
 
 	// handle is a key to uniquely identify this ImageOp
 	// in a map of cached textures.
-	handle interface{}
+	handle any
 }
 
 // ColorOp sets the brush to a constant color.
@@ -53,8 +53,7 @@ type LinearGradientOp struct {
 }
 
 // PaintOp fills the current clip area with the current brush.
-type PaintOp struct {
-}
+type PaintOp struct{}
 
 // OpacityStack represents an opacity applied to all painting operations
 // until Pop is called.