avoid calling a pure virtual function from GLBackend dtor

libraries/gpu-gl/src/gpu/gl/GLBackend.cpp

@@ -196,8 +196,6 @@ GLBackend::GLBackend() {
 
 
 GLBackend::~GLBackend() {
-    resetStages();
-
     killInput();
     killTransform();
 }

libraries/gpu-gl/src/gpu/gl/GLBackend.h

@@ -66,7 +66,7 @@ protected:
 public:
     static bool makeProgram(Shader& shader, const Shader::BindingSet& slotBindings = Shader::BindingSet());
 
-    ~GLBackend();
+    virtual ~GLBackend();
 
     void setCameraCorrection(const Mat4& correction);
     void render(const Batch& batch) final override;

libraries/gpu-gl/src/gpu/gl41/GL41Backend.h

@@ -43,6 +43,11 @@ public:
 
     explicit GL41Backend(bool syncCache) : Parent(syncCache) {}
     GL41Backend() : Parent() {}
+    virtual ~GL41Backend() {
+        // call resetStages here rather than in ~GLBackend dtor because it will call releaseResourceBuffer
+        // which is pure virtual from GLBackend's dtor.
+        resetStages();
+    }
 
     static const std::string GL41_VERSION;
     const std::string& getVersion() const override { return GL41_VERSION; }

libraries/gpu-gl/src/gpu/gl45/GL45Backend.h

@@ -39,6 +39,11 @@ public:
 
     explicit GL45Backend(bool syncCache) : Parent(syncCache) {}
     GL45Backend() : Parent() {}
+    virtual ~GL45Backend() {
+        // call resetStages here rather than in ~GLBackend dtor because it will call releaseResourceBuffer
+        // which is pure virtual from GLBackend's dtor.
+        resetStages();
+    }
 
     static const std::string GL45_VERSION;
     const std::string& getVersion() const override { return GL45_VERSION; }