Movement solver is insufficiently robust

This is an umbrella issue for behaviors fixed by the new movement solver but that don't have other issues filed.

• The movement solver doesn't actually slide across surfaces (it only does the vector rejection part, not the "get close to touching" part)
• Lack of a proper safety margin
• The bit of code in the movement solver that looks like "unstuck"ing doesn't actually function as such
• Actors don't unstuck themselves in general, which leads to problems with various vanilla entrances/exits that place actors slightly inside of geometry
• The movement solver gets snagged on sloped concave acute geometry easily (because bounces go back and forth between two nearby surfaces until it runs out of bounces)
• Various interactions involving jumping, stairs, slopes, etc. do not function the same way as they do in vanilla