make spinner volume at its start, preserve slider colours, and normalise fountain-shoot.wav

[nominomu727]

hi! sorry if these three issues should be in separate discussions, i'm not exactly sure how this exactly works.

make the volume-adjusting inherited timing point of spinners at the start (or end) of the spinner

currently, when a spinner's volume in lazer is different to the volume of the previous object, it'll generate a green line in the stable-compatible .osu to change the volume accordingly, but this green line occurs in the middle of the spinner (intuitively it should be the beginning or the end, to avoid a sudden volume change in the middle of the spinner)

this actually led to this issue in beatmap modding: https://osu.ppy.sh/beatmapsets/2354986/discussion/-/generalAll#/5552429

i made a test map to demonstrate this, you can download it here: https://files.catbox.moe/d8ltr8.olz

and here's what it looks like in lazer:

and here's what it looks like in stable!

preserve sliderborder and slidertrackoverride colours

saving a map in lazer wipes custom sliderborder and slidertrackoverride colours. in fact, the original test map from above was actually this, before i saved the map:

normalise fountain-shoot.wav

the default fountain-shoot.wav sample is quite loud and a bit of a jumpscare on quieter songs. i think it'd be nice if its volume was normalised based on the RMS volume of the song at the time.

thank you for reading and considering !!