Add proper slider rendering

[LeaPhant]

Currently the slider rendering lacks catmull sliders and bezier sliders are poorly implemented. Slider repeats aren't implemented visually either. I need help for this as I'm really bad at math/rendering so feel free to PR.

Here are references from the osu!lazer codebase:

ppy/osu/osu.Game/Rulesets/Objects/SliderPath.cs
ppy/osu-framework/osu.Framework/MathUtils/PathApproximator.cs

[LeaPhant]

Update on this: I added catmull sliders and circular arc sliders, bezier sliders could still be optimized but it's fine for now. Also I finally got slider point interpolation so every point on a slider track always the same distance, this prevents any sudden velocity changes and should result in always smooth linear movement of the slider ball (this is huge). Now that slider tracks are cleaned up slider repeats are next.

[LeaPhant]

Update: Slider repeats have been added, slider ticks are mostly done but I still have to find the algorithm which osu uses to decide which slider ticks to dismiss. Also slider velocity is still buggy and some sliders are too slow and therefore too long in time, but this is a fix to do in my osu-parser fork.

[LeaPhant]

Slider velocity is finally fixed in the osu-parser fork, slider length should be accurate now.

[LeaPhant]

Update: Slider ticks are finally correct and with that I'm pretty sure sliders are basically done. Now only minor changes like performance improvements or adding a gradient to slider bodies are left.