Celeste Type has bug in sliding

[LugeKit]

I have tried out the demo and find that the player is quite strange using Celeste Type.
After a short debugging time, I think the Celeste Type's SlideSpeed && SlideAccel should not be zero.
Becuase I am using a windows, I cannot make a pull request to fix this.

Also, I think the implementation of slide is buggy. If I make a double wall jump in level 2, I will slide upwards a lot on the second wall, which is not intended. Maybe letting the velocity.y be zero at starting frame of wall sliding is a good idea.