dx/dy/dz check for feet origin rather than intersecting hitbox

Partner Rank: 3

Verification builds: 1.13.0.2, 1.12, 1.2.20.1
Regression builds: None, this has always been broken (since 1.2, earliest I can check)

Summary: The dx, dy, and dz options in selectors are supposed to check if any part of an entity's hitbox intersects with a box region. However, instead they check if the origin point at the entity's feet intersects with the region.

Impact: This prevents a small mechanic in an upcoming map.

Repro Steps:

1. /execute @p ~ ~1 ~ say @s[dy=1]

Expected Results: Player is found, since your hitbox intersects with the box that starts in your waist.

Observed Results: Player is not found.

Screenshots/Videos attached: No

Notes: Very similar issue is that these options throw an error when using a non-whole number, e.g. @a[dx=0.5] should work, but instead invalidates the command. I'll create a new ticket for this if necessary.