[Bug]: Avatar input controls don't respect ground slope

[speigg]

Currently we are translating our 2D input controls to avatar movement velocity in the x/z plane, which causes several potential issues:

• walking up any slope is slower
• walking down any slope means you basically skip/hop on your way down (because your velocity is straight forward off the ground, and then you fall/float down repeatedly applying velocity directly forward)

We should probably be applying velocity directly along the surface of the current slope