io/pointer: [API] make cursor name into a byte

Add most of the common cursors defined by different systems.

Normalize cursor names to match CSS.

This is API change: some cursor names have changed, and the
underlying type is no longer a string.

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

op/clip/clip_test.go

@@ -37,7 +37,7 @@ func TestPathBegin(t *testing.T) {
 	ops := new(op.Ops)
 	var p clip.Path
 	p.Begin(ops)
-	p.LineTo(f32.Point{10, 10})
+	p.LineTo(f32.Pt(10, 10))
 	p.Close()
 	stack := clip.Outline{Path: p.End()}.Op().Push(ops)
 	paint.Fill(ops, color.NRGBA{A: 255})