Fix cast_motion sometimes failing

- Fixes Godot physics failing when the cast Shape is inside of, or
already colliding with another Shape.
- Fixes Bullet physics failing when there is no motion.
- Ensures Godot and Bullet physics behave the same.
- Updates the documentation to exclude the caveats for the failures and
differences.

doc/classes/PhysicsDirectSpaceState2D.xml

@@ -16,8 +16,9 @@
 			<argument index="0" name="shape" type="PhysicsShapeQueryParameters2D">
 			</argument>
 			<description>
-				Checks how far the shape can travel toward a point. If the shape can not move, the array will be empty.
-				[b]Note:[/b] Both the shape and the motion are supplied through a [PhysicsShapeQueryParameters2D] object. The method will return an array with two floats between 0 and 1, both representing a fraction of [code]motion[/code]. The first is how far the shape can move without triggering a collision, and the second is the point at which a collision will occur. If no collision is detected, the returned array will be [code][1, 1][/code].
+				Checks how far a [Shape2D] can move without colliding. All the parameters for the query, including the shape and the motion, are supplied through a [PhysicsShapeQueryParameters2D] object.
+				Returns an array with the safe and unsafe proportions (between 0 and 1) of the motion. The safe proportion is the maximum fraction of the motion that can be made without a collision. The unsafe proportion is the minimum fraction of the distance that must be moved for a collision. If no collision is detected a result of [code][1.0, 1.0][/code] will be returned.
+				[b]Note:[/b] Any [Shape2D]s that the shape is already colliding with e.g. inside of, will be ignored. Use [method collide_shape] to determine the [Shape2D]s that the shape is already colliding with.
 			</description>
 		</method>
 		<method name="collide_shape">

doc/classes/PhysicsDirectSpaceState3D.xml

@@ -18,8 +18,9 @@
 			<argument index="1" name="motion" type="Vector3">
 			</argument>
 			<description>
-				Checks whether the shape can travel to a point. The method will return an array with two floats between 0 and 1, both representing a fraction of [code]motion[/code]. The first is how far the shape can move without triggering a collision, and the second is the point at which a collision will occur. If no collision is detected, the returned array will be [code][1, 1][/code].
-				If the shape can not move, the returned array will be [code][0, 0][/code] under Bullet, and empty under GodotPhysics3D.
+				Checks how far a [Shape3D] can move without colliding. All the parameters for the query, including the shape, are supplied through a [PhysicsShapeQueryParameters3D] object.
+				Returns an array with the safe and unsafe proportions (between 0 and 1) of the motion. The safe proportion is the maximum fraction of the motion that can be made without a collision. The unsafe proportion is the minimum fraction of the distance that must be moved for a collision. If no collision is detected a result of [code][1.0, 1.0][/code] will be returned.
+				[b]Note:[/b] Any [Shape3D]s that the shape is already colliding with e.g. inside of, will be ignored. Use [method collide_shape] to determine the [Shape3D]s that the shape is already colliding with.
 			</description>
 		</method>
 		<method name="collide_shape">

modules/bullet/space_bullet.cpp

@@ -177,8 +177,10 @@ bool BulletPhysicsDirectSpaceState::cast_motion(const RID &p_shape, const Transf
 	bt_xform_to.getOrigin() += bt_motion;
 
 	if ((bt_xform_to.getOrigin() - bt_xform_from.getOrigin()).fuzzyZero()) {
+		r_closest_safe = 1.0f;
+		r_closest_unsafe = 1.0f;
 		bulletdelete(btShape);
-		return false;
+		return true;
 	}
 
 	GodotClosestConvexResultCallback btResult(bt_xform_from.getOrigin(), bt_xform_to.getOrigin(), &p_exclude, p_collide_with_bodies, p_collide_with_areas);

servers/physics_2d/space_2d_sw.cpp

@@ -278,9 +278,9 @@ bool PhysicsDirectSpaceState2DSW::cast_motion(const RID &p_shape, const Transfor
 			continue;
 		}
 
-		//test initial overlap
+		//test initial overlap, ignore objects it's inside of.
 		if (CollisionSolver2DSW::solve(shape, p_xform, Vector2(), col_obj->get_shape(shape_idx), col_obj_xform, Vector2(), nullptr, nullptr, nullptr, p_margin)) {
-			return false;
+			continue;
 		}
 
 		//just do kinematic solving

servers/physics_3d/space_3d_sw.cpp

@@ -274,11 +274,11 @@ bool PhysicsDirectSpaceState3DSW::cast_motion(const RID &p_shape, const Transfor
 			continue;
 		}
 
-		//test initial overlap
+		//test initial overlap, ignore objects it's inside of.
 		sep_axis = p_motion.normalized();
 
 		if (!CollisionSolver3DSW::solve_distance(shape, p_xform, col_obj->get_shape(shape_idx), col_obj_xform, point_A, point_B, aabb, &sep_axis)) {
-			return false;
+			continue;
 		}
 
 		//just do kinematic solving