Added virtual destructor to FPresentShaderBase class

No more 'delete called on non-final class that has virtual functions but non-virtual destructor' warnings reported by GCC/Clang

src/gl/shaders/gl_presentshader.h

@@ -6,6 +6,7 @@
 class FPresentShaderBase
 {
 public:
+	virtual ~FPresentShaderBase() {}
 	virtual void Bind() = 0;
 
 	FBufferedUniform1f InvGamma;