Refactors immerse element to use alpha filters instead of static overlays (#93038)

## About The Pull Request
Immerse element now uses an alpha filter rather than a vis_contents
object, which allows them to be much more smooth and seamless. There's
no longer a visible contour on fully opaque liquids, nor a janky effect
when you move in a liquid. This also fixes the broken fluid animation,
so now it actually has a bit of a wave to it.

<img width="179" height="183" alt="dreamseeker_PDjP1zyMRl"
src="https://github.com/user-attachments/assets/7c1bbefe-0e97-456e-a303-c34e6a1a238a"
/>
<img width="177" height="180" alt="dreamseeker_hGjKOyBL8f"
src="https://github.com/user-attachments/assets/6c3bc33f-a22c-452a-beb0-9dd44b080a7c"
/>

<img width="152" height="162" alt="dreamseeker_Et3eRd3NF6"
src="https://github.com/user-attachments/assets/1478aaba-d345-44de-8baa-9d0da0bc9d1c"
/>
<img width="185" height="182" alt="dreamseeker_5Iok1lUni2"
src="https://github.com/user-attachments/assets/4ac5fea4-24a7-46c2-b475-4445a43493b4"
/>

The code is immensely cursed in some places, ideally this should not
have to use vis_contents whatsoever but BYOND seems to be intent on
causing memory leaks whenever you try to set mutable's render_target to
an interpolated string, so I'm using a VIS_HIDE object as a relay for
the filter for the time being.
I've ended up changing some mob pixel_y offsets to pixel_z (as they
should've been from the start) to account for this (the logic is being
that pixel_y is "physical" position on the turf, while pixel_z is how
high above the turf something is)

## Why It's Good For The Game

The effect is less jank and looks cool.

## Changelog
🆑
refactor: Refactors immerse element to use alpha filters instead of
static overlays. It should look much prettier now.
/🆑

code/datums/components/riding/riding.dm

@@ -185,7 +185,8 @@
 	if(seat.buckle_lying && rider.body_position == LYING_DOWN)
 		y_offset += (-1 * PIXEL_Y_OFFSET_LYING)
 
-	rider.add_offsets(RIDING_SOURCE, x_add = x_offset, y_add = y_offset, animate = animate)
+	// Rider uses pixel_z offsets as they're above the turf, not up north on the turf
+	rider.add_offsets(RIDING_SOURCE, x_add = x_offset, z_add = y_offset, animate = animate)
 	rider.layer = layer
 
 #undef GET_X_OFFSET