Fixing audio glitches with wave and noise channels

include/ppu.h

@@ -7,8 +7,6 @@ static const int TICKS_PER_LINE = 456;
 static const int YRES = 144;
 static const int XRES = 160;
 
-static u32 target_frame_time;
-
 bool window_visible();
 
 typedef enum {
@@ -81,6 +79,7 @@ typedef struct {
     u32 *video_buffer;
 
     bool rendering_window;
+    u32 target_frame_time;
 } ppu_context;
 
 void ppu_init();