[Maintenance] [th04/th05] Declare scrolling variables and functions in C land

Part of P0089, funded by [Anonymous] and Blue Bolt.

th04/main/scroll.hpp

@@ -0,0 +1,24 @@
+// [scroll_line] is advanced by 1 for every 16 units.
+extern SubpixelLength8 scroll_subpixel_line;
+
+// Amount to add to [scroll_subpixel_line] every frame.
+extern SubpixelLength8 scroll_speed;
+
+// Current line at the top of VRAM.
+extern int scroll_line;
+extern int scroll_line_on_page[2];
+
+// Playfield-space pixels scrolled in the last frame.
+extern Subpixel scroll_last_delta;
+
+// If false, the game doesn't draw stage tiles, assuming that someone else
+// draws the background.
+extern bool scroll_active;
+
+// Transforms [y] to its corresponding VRAM line, adding the current
+// [scroll_line] or 0 if scrolling is disabled.
+int pascal near scroll_subpixel_y_to_vram_seg1(subpixel_t y);
+int pascal near scroll_subpixel_y_to_vram_seg3(subpixel_t y);
+// Like the one above, but always adds [scroll_line], even if scrolling is
+// disabled.
+int pascal near scroll_subpixel_y_to_vram_always(subpixel_t y);

th04/main/scroll[bss].asm

@@ -1,12 +1,7 @@
 public _scroll_subpixel_line, _scroll_speed, _scroll_line, _scroll_active
-; [scroll_line] is advanced by 1 for every 16 units.
 _scroll_subpixel_line	db ?
-; Amount to add to [_scroll_subpixel_line] every frame.
 _scroll_speed	db ?
-; Current line at the top of VRAM.
 _scroll_line	dw ?
-; Amount of 1/16th-pixel units scrolled in the last frame.
 _scroll_last_delta	dw ?
-; If set to 0, the game assumes that the background is drawn by someone else!
 _scroll_active	db ?
-		db ?
+	evendata

th04/main/scroll_y_1.asm

@@ -1,7 +1,3 @@
-; Transforms the 1/16-pixel on-screen Y coordinate [sy] to its corresponding
-; VRAM line given the current scrolling offset.
-
-; int pascal near scroll_subpixel_y_to_vram(int sy);
 public SCROLL_SUBPIXEL_Y_TO_VRAM_SEG1
 scroll_subpixel_y_to_vram_seg1	proc near
 @@sy	= word ptr 2

th04/main/scroll_y_3.asm

@@ -1,4 +1,3 @@
-; See scroll_subpixel_y_to_vram_seg1().
 public SCROLL_SUBPIXEL_Y_TO_VRAM_SEG3
 scroll_subpixel_y_to_vram_seg3	proc near
 @@sy	= word ptr 2
@@ -26,7 +25,6 @@ scroll_subpixel_y_to_vram_seg3	proc near
 scroll_subpixel_y_to_vram_seg3	endp
 
 
-; Like the one above, but transforming even if scrolling is disabled.
 public SCROLL_SUBPIXEL_Y_TO_VRAM_ALWAYS
 scroll_subpixel_y_to_vram_always	proc near
 @@sy	= word ptr 2

th04/main/tiles_invalidate.asm

@@ -53,7 +53,7 @@ tiles_invalidate_around	proc near
 	mov	bh, 0
 	mov	bl, _page_back
 	add	bx, bx
-	add	ax, _scroll_line_on_plane[bx]
+	add	ax, _scroll_line_on_page[bx]
 	jns	short @@scroll_wrap?
 	add	ax, RES_Y
 	jmp	short @@invalidate

th04/main/tiles_invalidate[bss].asm

@@ -1,3 +1,3 @@
-public _scroll_line_on_plane, _tile_invalidate_box
-_scroll_line_on_plane	dw 2 dup(?)
+public _scroll_line_on_page, _tile_invalidate_box
+_scroll_line_on_page	dw 2 dup(?)
 _tile_invalidate_box	Point <?>

th04_main.asm

@@ -920,8 +920,8 @@ sub_B1D0	proc near
 		mov	byte_2D00A, 0
 		mov	_scroll_line, 0
 		mov	word_25100, 0
-		mov	_scroll_line_on_plane[0 * 2], 0
-		mov	_scroll_line_on_plane[1 * 2], 0
+		mov	_scroll_line_on_page[0 * 2], 0
+		mov	_scroll_line_on_page[1 * 2], 0
 		mov	_scroll_subpixel_line, 0
 		mov	byte_25104, 0
 		mov	byte_250FE, 0
@@ -2516,7 +2516,7 @@ sub_CCD6	proc near
 		add	ax, ax
 		mov	dx, _scroll_line
 		mov	bx, ax
-		mov	_scroll_line_on_plane[bx], dx
+		mov	_scroll_line_on_page[bx], dx
 		cmp	byte_250FE, 0
 		jz	short loc_CCFE
 		cmp	_scroll_active, 0

th05_main.asm

@@ -1074,8 +1074,8 @@ sub_B55A	proc near
 		mov	byte_2C96C, 0
 		mov	_scroll_line, 0
 		mov	word_23F06, 0
-		mov	_scroll_line_on_plane[0 * 2], 0
-		mov	_scroll_line_on_plane[1 * 2], 0
+		mov	_scroll_line_on_page[0 * 2], 0
+		mov	_scroll_line_on_page[1 * 2], 0
 		mov	_scroll_subpixel_line, 0
 		mov	byte_23EFC, 0
 		mov	byte_23F04, 0
@@ -7473,7 +7473,7 @@ loc_10223:
 		add	ax, ax
 		mov	dx, _scroll_line
 		mov	bx, ax
-		mov	_scroll_line_on_plane[bx], dx
+		mov	_scroll_line_on_page[bx], dx
 		cmp	byte_23F04, 0
 		jz	short loc_1024F
 		cmp	_scroll_active, 0