Fix new GCC 9 warnings: -Wdeprecated-copy.

(cherry picked from commit 6be77da7eb)

core/math/audio_frame.h

@@ -122,6 +122,12 @@ struct AudioFrame {
 		r = p_frame.r;
 	}
 
+	_ALWAYS_INLINE_ AudioFrame operator=(const AudioFrame &p_frame) {
+		l = p_frame.l;
+		r = p_frame.r;
+		return *this;
+	}
+
 	_ALWAYS_INLINE_ AudioFrame() {}
 };
 

core/math/quat.h

@@ -131,6 +131,14 @@ public:
 			w(q.w) {
 	}
 
+	Quat operator=(const Quat &q) {
+		x = q.x;
+		y = q.y;
+		z = q.z;
+		w = q.w;
+		return *this;
+	}
+
 	Quat(const Vector3 &v0, const Vector3 &v1) // shortest arc
 	{
 		Vector3 c = v0.cross(v1);

core/ustring.h

@@ -97,6 +97,10 @@ public:
 
 	_FORCE_INLINE_ CharString() {}
 	_FORCE_INLINE_ CharString(const CharString &p_str) { _cowdata._ref(p_str._cowdata); }
+	_FORCE_INLINE_ CharString operator=(const CharString &p_str) {
+		_cowdata._ref(p_str._cowdata);
+		return *this;
+	}
 
 	bool operator<(const CharString &p_right) const;
 	CharString &operator+=(char p_char);
@@ -339,6 +343,10 @@ public:
 
 	_FORCE_INLINE_ String() {}
 	_FORCE_INLINE_ String(const String &p_str) { _cowdata._ref(p_str._cowdata); }
+	String operator=(const String &p_str) {
+		_cowdata._ref(p_str._cowdata);
+		return *this;
+	}
 
 	String(const char *p_str);
 	String(const CharType *p_str, int p_clip_to_len = -1);