[feat]: Filter (#18)

closes #16

Reviewed-on: #18

inc/SynthGuiState.h

@@ -2,6 +2,7 @@
 #include "OscillatorType.h"
 #include "raygui.h"
 #include <vector>
+#include "Filter.h"
 
 struct OscillatorGuiState {
     float volume;
@@ -18,7 +19,15 @@ struct ADSRGuiState {
     float release;
 };
 
+struct FilterGuiState {
+    float freq;
+    float res; //todo: res
+    FilterType type;
+    bool is_dropdown_open;
+};
+
 struct SynthGuiState {
     std::vector<OscillatorGuiState*> oscillators;
     ADSRGuiState adsr;
+    FilterGuiState filter;
 };