Fix collisions when moving from one-way to slopes

[matt-flaig]

When moving from one-way tiles to upward slopes, entities would get stuck at the edge of the slope and require upwards momentum to be able to continue moving forwards.

Here's a sample entity trying to move up a slope from a one-way tile, previously:

Here's the entity freely moving up the slope with this fix:

[finscn]

Glad to see someone is still watching impact.js .

I think you are an expert in impact.js area .

Do you tell me something about this #99 ? Thanks.

[tempral]

I merged your solution and added some slopes in the test.js map (box2d physics demo)
but the player still doesn't walk up neither down slopes