Done for two reasons:
- During review it was requested for the logic to be moved out of
`BezierConverter` as `BezierConverter` was intended to produce
"lazer style" sliders with per-control-point curve types,
as a future usability / code layering concern.
- It is also relevant for encode-decode stability. With how the logic
was structured between the Bezier converter and the legacy beatmap
encoder, the encoder would leave behind per-control-point Bezier curve
specs that stable ignored, but subsequent encodes and decodes in lazer
would end up multiplying the doubled-up control points ad nauseam.
Instead, it is sufficient to only specify the curve type for the
head control point as Bezier, not specify any further curve types
later on, and instead just keep the double-up-control-point for new
implicit segment logic which is enough to make stable cooperate
(and also as close to outputting the slider exactly as stable would
have produced it as we've ever been)
