SOUND: multiple Wave channel fixes and GB/GBC behavior

* SOUND: channel 3 read dependent on current wave index

* SOUND: maintain wave_trigger high for longer, passes blargg sound test 9 for gbc, added is_gbc input to gbc_snd

* SOUND: added wave table access timewindow in DMG mode, outside of it the read returns 0xFF when the sound is playing

* gbc_snd.vhd whitespaces cleanup

* SOUND: refactored write process to handle reset and specific DMG behaviour (length counter writable when APU is off)
SOUND: initialized wavetable with known values, fixes aladdin and R-type now behaves the same as real hardware

* SOUND: refactored sound processing vhdl process
SOUND: sq1 and sq2 duty cycles are not accessible when the APU is off in DMG mode(only the length counters)

rtl/gb.v

@@ -236,6 +236,8 @@ gbc_snd audio (
 	.clk				( clk_sys			),
 	.ce            ( ce_2x           ),
 	.reset			( reset				),
+	
+	.is_gbc        ( isGBC           ),
 
 	.s1_read  		( audio_rd  		),
 	.s1_write 		( audio_wr  		),