[Maintenance] Assign segment-specific names to the motion_update() variants

Well, we *could* pointlessly decompile this function into an unreadable
mess, but only if we throw away the semantics of the return value, and
replace its type with `void` or `uint32_t`.
(Turbo C++ can't cast registers to a struct value, and adding a
constructor to SPPoint would break everything else, if it even works.)
ZUN's original usage code doesn't care either way, because it only ever
accesses the returned value directly through DX:AX.

Part of P0148, funded by [Anonymous].

Author: nmlgc

th04/main/player/pos_update_and_clamp.asm

@@ -2,7 +2,7 @@ public PLAYER_POS_UPDATE_AND_CLAMP
 player_pos_update_and_clamp	proc near
 	push	bp
 	mov	bp, sp
-	call	_motion_update_1 pascal, offset _player_pos
+	call	motion_update_seg1 pascal, offset _player_pos
 	cmp	ax, ((0 + PLAYER_MOVE_MARGIN_LEFT) shl 4)
 	jge	short @@clamp_right?
 	mov	ax, ((0 + PLAYER_MOVE_MARGIN_LEFT) shl 4)

th04/main/sparks.asm

@@ -17,7 +17,7 @@ sparks_update	proc near
 @@update:
 	lea	ax, [si+spark_t.pos]
 	push	ax
-	call	_motion_update_1
+	call	motion_update_seg1
 	add	ax, ((SPARK_W / 2) shl 4)
 	cmp	ax, ((PLAYFIELD_W + SPARK_W) shl 4)
 	jnb	short @@remove

th04/math/motion.hpp

@@ -10,3 +10,11 @@ typedef struct {
 		prev.y.set(screen_y);
 	}
 } motion_t;
+
+// Moves the given [motion] instance by one step, and returns its new position.
+// The _seg1() and _seg3() variants are provided for near calls within code
+// segment #1 (rendering) and code segment #3 (update), respectively.
+// Note that some calls to these functions directly access the returned value
+// through the AX (X) and DX (Y) registers.
+SPPoint pascal near motion_update_seg1(motion_t near &motion);
+SPPoint pascal near motion_update_seg3(motion_t near &motion);

th04/math/motion.inc

@@ -1,16 +1,12 @@
-; For increased precision, all values are stored in units of 1/16th of a pixel.
 motion_t	struc
 	cur      	Point ?
 	prev    	Point ?
 	velocity	Point ?
 motion_t	ends
 
 MOTION_UPDATE_DEF macro instance
-	; Returns the new current position in AX (X) and DX (Y).
-	;
-	; Point pascal near motion_update(motion_t near *motion);
-	public _MOTION_UPDATE_&instance
-	_motion_update_&instance proc near
+	public MOTION_UPDATE_SEG&instance
+	motion_update_seg&instance proc near
 	arg_bx	near, @motion:word
 
 		mov	bx, @motion
@@ -24,5 +20,5 @@ MOTION_UPDATE_DEF macro instance
 		add	dx, word ptr [bx+motion_t.velocity]
 		mov	word ptr [bx+motion_t.cur], dx
 		ret_bx
-	_motion_update_&instance endp
+	motion_update_seg&instance endp
 endm