Remove unneeded includes

Some are harmless, some screw with SDL's standard includes due to preceeding `_USE_MATH_DEFINES`.

src/engine/engine.cpp

@@ -17,9 +17,6 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#include "dataErrors.h"
-#include "song.h"
-#include <functional>
 #define _USE_MATH_DEFINES
 #include "engine.h"
 #include "instrument.h"

src/engine/filter.cpp

@@ -17,7 +17,6 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#include <stddef.h>
 #define _USE_MATH_DEFINES
 #include <math.h>
 #include "filter.h"
@@ -85,4 +84,4 @@ float* DivFilterTables::getSincIntegralTable() {
     }
   }
   return sincIntegralTable;
-}
+}

src/engine/playback.cpp

@@ -17,9 +17,6 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#include "blip_buf.h"
-#include "song.h"
-#include "wavetable.h"
 #define _USE_MATH_DEFINES
 #include "dispatch.h"
 #include "engine.h"

src/gui/pattern.cpp

@@ -17,8 +17,6 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#include <SDL_timer.h>
-#include <imgui.h>
 #define _USE_MATH_DEFINES
 #include "gui.h"
 #include "../ta-log.h"