Unexpected behavior of `RotationStyle`

[nighca]

In Scratch, for a sprite with rotationStyle: left-right:

• heading: [0, 180] is considered facing right
• heading: (-180, 0) is considered facing left

While in spx, the logic is strange:

https://github.com/goplus/spx/blob/72b5b4f2242e9cbfd9017ceba3119d581b7a72e3/spgdi.go#L72-L79

I don't understand. But it causes some result that is IMO not expected, for example:

setRotationStyle LeftRight

setHeading float64(Left)  // spx will render the sprite facing right
setHeading float64(Down)  // spx will render the sprite facing left
setHeading float64(Right) // spx will render the sprite facing right
setHeading float64(Up)    // spx will render the sprite facing right

I think it is reasonable to adopt the same logic as Scratch.

As discussed, we should adopt the same logic as Scratch.

[nighca]

fixed by #303