Add is_available function to CD-ROM devices

Now `SI 2` command returns zero if no audio track is available.

src/cdrom.bgm.c

@@ -63,6 +63,10 @@ static bool cdrom_bgm_getPlayingInfo(cd_time *info) {
 	return true;
 }
 
+static bool cdrom_bgm_is_available(void) {
+	return true;
+}
+
 cdromdevice_t cdrom_bgm = {
 	.init = cdrom_bgm_init,
 	.exit = cdrom_bgm_exit,
@@ -70,4 +74,5 @@ cdromdevice_t cdrom_bgm = {
 	.start = cdrom_bgm_start,
 	.stop = cdrom_bgm_stop,
 	.getpos = cdrom_bgm_getPlayingInfo,
+	.is_available = cdrom_bgm_is_available,
 };

src/cdrom.emscripten.c

@@ -62,6 +62,10 @@ static bool cdrom_getPlayingInfo(cd_time *info) {
 	return true;
 }
 
+static bool cdrom_is_available(void) {
+	return EM_ASM_INT({ return xsystem35.cdPlayer.hasAudioTrack(); });
+}
+
 cdromdevice_t cdrom_emscripten = {
 	.init = cdrom_init,
 	.exit = cdrom_exit,
@@ -69,4 +73,5 @@ cdromdevice_t cdrom_emscripten = {
 	.start = cdrom_start,
 	.stop = cdrom_stop,
 	.getpos = cdrom_getPlayingInfo,
+	.is_available = cdrom_is_available,
 };

src/cdrom.h

@@ -38,6 +38,7 @@ typedef struct {
 	bool (*start)(int trk, int loop);
 	void (*stop)(void);
 	bool (*getpos)(cd_time *);
+	bool (*is_available)(void);
 } cdromdevice_t;
 
 extern cdromdevice_t cdrom_bgm;

src/cdrom.mp3.c

@@ -164,6 +164,10 @@ static bool cdrom_getPlayingInfo (cd_time *inf) {
 	return true;
 }
 
+static bool cdrom_is_available(void) {
+	return enabled;
+}
+
 cdromdevice_t cdrom_mp3 = {
 	.init = cdrom_init,
 	.exit = cdrom_exit,
@@ -171,4 +175,5 @@ cdromdevice_t cdrom_mp3 = {
 	.start = cdrom_start,
 	.stop = cdrom_stop,
 	.getpos = cdrom_getPlayingInfo,
+	.is_available = cdrom_is_available,
 };

src/music_cdrom.c

@@ -113,5 +113,5 @@ int muscd_get_maxtrack(void) {
 }
 
 bool muscd_is_available(void) {
-	return prv.cddev;
+	return prv.cddev && prv.cddev->is_available();
 }