chiptune: add disable rasterbars support (R key)

chiptune_player/README.chiptune

@@ -4,6 +4,7 @@ Challenges found writing an Apple II chiptune player
 
   http://www.deater.net/weave/vmwprod/chiptune/
 ====================================================
+		27 February 2018
 
 GOAL:
 ~~~~~
@@ -21,7 +22,10 @@ USING:
    Applewin works fine (even under Wine on Linux).
    MESS does too, it's harder to setup (ROMs) but the audio sounds clearer.
 
-   Space pauses, Left/Right arrow switches songs.
+   Key bindings:
+	Spacebar -- pauses
+	Left/Right arrow -- switches songs
+	R -- enables/disables rasterbars
 
    You can load up your own YM5 files.  Get the "ym5_to_krw" utility found in
    the repository https://github.com/deater/vmw-meter/

chiptune_player/chiptune_player.s

@@ -26,6 +26,9 @@ NUM_FILES	EQU	15
 	sta	MB_CHUNK_OFFSET
 	sta	DECODE_ERROR
 
+	lda	#$ff
+	sta	RASTERBARS_ON
+
 	lda	#14				; start at WAVE
 	sta	WHICH_FILE
 

chiptune_player/interrupt_handler.s

@@ -266,7 +266,10 @@ done_interrupt:
 	;============================
 
 	jsr	clear_top
+	lda	RASTERBARS_ON
+	beq	skip_rasters
 	jsr	draw_rasters
+skip_rasters:
 	jsr	volume_bars
 	jsr	page_flip
 
@@ -278,12 +281,21 @@ check_keyboard:
 	beq	exit_interrupt
 
 	cmp	#(' '+$80)
-	bne	key_left
+	bne	key_R
 key_space:
 	lda	#$80
 	eor	DONE_PLAYING
 	jmp	quiet_exit
 
+key_R:
+	cmp	#'R'
+	bne	key_left
+
+	lda	#$ff
+	eor	RASTERBARS_ON
+	sta	RASTERBARS_ON
+	jmp	done_key
+
 key_left:
 	cmp	#'A'
 	bne	key_right

chiptune_player/zp.inc

@@ -109,6 +109,7 @@ WHICH_FILE	EQU $8A
 				;LAST_MAP_COLOR	EQU $89
 				;DRAW_SKY	EQU $8A
 COLOR_MASK	EQU $8B
+RASTERBARS_ON	EQU $8C
 				;KEY_COUNT	EQU $8C
 				;KEY_OFFSET	EQU $8D
 				;DRAW_BLUE_SKY	EQU $8E