i3dl2/reverb: Mark relevant member functions as const

These two don't modify member state.

src/audio_core/renderer/effect/i3dl2.h

@@ -99,7 +99,7 @@ public:
             return out_sample;
         }
 
-        Common::FixedPoint<50, 14> Read() {
+        Common::FixedPoint<50, 14> Read() const {
             return *output;
         }
 
@@ -110,7 +110,7 @@ public:
             }
         }
 
-        Common::FixedPoint<50, 14> TapOut(const s32 index) {
+        Common::FixedPoint<50, 14> TapOut(const s32 index) const {
             auto out{input - (index + 1)};
             if (out < buffer.data()) {
                 out += max_delay + 1;

src/audio_core/renderer/effect/reverb.h

@@ -95,7 +95,7 @@ public:
             return out_sample;
         }
 
-        Common::FixedPoint<50, 14> Read() {
+        Common::FixedPoint<50, 14> Read() const {
             return *output;
         }
 
@@ -106,7 +106,7 @@ public:
             }
         }
 
-        Common::FixedPoint<50, 14> TapOut(const s32 index) {
+        Common::FixedPoint<50, 14> TapOut(const s32 index) const {
             auto out{input - (index + 1)};
             if (out < buffer.data()) {
                 out += sample_count;