Fix all horizontal movement being broken

[Macil]

Fixes #1141.

The commit 940e53a seems to have broken all horizontal movement in Raze. The commit copied a change to vectors.h from GZDoom that makes it so TVector3::XY() returns a fresh TVector2 object instead of a reference to the TVector3 type-punned to look like a TVector2, which has the consequence that modifying the returned object (like vec3.XY() = someVec2;) no longer works to modify the TVector3. That wasn't an issue in GZDoom but this pattern is done in a ton of places in Raze.

The commit changed vectors.h to no longer use type-punning which seems like a reasonable goal, so I wanted to find a solution that didn't just revert that. I think it would be possible to make all of Raze's code continue working by making the TVector3::XY() method return a TVector2-like proxy object that contains references to the TVector3's X and Y properties and implements all of the same methods as TVector2 and support for being implicitly converted to a TVector2, but that struck me as a lot of duplication in vectors.h which also wouldn't be useful for GZDoom. Instead I just went through all of the calls to TVector3::XY() that mutated the returned value and made them not do that.

Here are the main kinds of changes I did many times:

-spr.pos.XY() = opos.XY();
+spr.pos.SetXY(opos.XY());

I added a TVector3::SetXY(const Vector2 &v) method instead of overloading operator= for Vector2 values because it seemed like the latter might allow accidents.

-pPlayerActor->vel.XY().Zero();
+pPlayerActor->vel.SetXY(DVector2(0, 0));

-tspr->pos.XY() += tspr->Angles.Yaw.ToVector() * 0.125;
+tspr->pos += tspr->Angles.Yaw.ToVector() * 0.125;

The above works because TVector3 has methods like TVector3 &operator+= (const Vector2 &other) and other compound assignment operators defined for it that only affects the X and Y properties.

-p->vel.XY() *= gs.playerfriction;
+p->vel.SetXY(p->vel.XY() * gs.playerfriction);

This had to be handled differently because gs.playerfriction is a number instead of a Vector2, so if I just removed the .XY() part again then all 3 coordinates would be modified, not just X and Y.

Additionally, while working on this I discovered a bug in source/games/blood/src/aiunicult.cpp dudeLeechOperate() where the Z coordinate of a vector wasn't set to the intended value and was left uninitialized. It's fixed in its own commit, 3ff9ef5.