[GL3+] Fix destruction of RenderTextures destroying regular Textures

destroyInternalResourcesImpl was only being called from within
GL3PlusTextureGpu's destructor.

However it is a virtual call, and thus
GL3PlusTextureGpuRenderTarget::destroyInternalResourcesImpl would never
get called.

This led GL3PlusTextureGpu::destroyInternalResourcesImpl to believe
there is a regular Texture in mFinalTextureName, because derived class
never got to zero it out.

To trigger this scenario:
* The RenderTexture had to be a depth buffer or be created with the
NotTexture flag
* The TextureGpu must be destroyed without having been transitioned to
non-resident
* A regular texture and a RenderTarget had both been asigned the same
mFinalTextureName handle value (one is a glRenderbuffer, the other a
glTexture)

Although the conditions are hard to trigger, the fix for shared depth
buffers leaking (#103) skyrocketed the likehood of encountering this bug

RenderSystems/GL3Plus/include/OgreGL3PlusTextureGpu.h

@@ -147,6 +147,7 @@ namespace Ogre
                                        VaoManager *vaoManager, IdString name, uint32 textureFlags,
                                        TextureTypes::TextureTypes initialType,
                                        TextureGpuManager *textureManager );
+        virtual ~GL3PlusTextureGpuRenderTarget();
 
         virtual void _setDepthBufferDefaults( uint16 depthBufferPoolId, bool preferDepthTexture,
                                               PixelFormatGpu desiredDepthBufferFormat );

RenderSystems/GL3Plus/src/OgreGL3PlusTextureGpu.cpp

@@ -744,6 +744,11 @@ namespace Ogre
             mDepthBufferPoolId = 0;
     }
     //-----------------------------------------------------------------------------------
+    GL3PlusTextureGpuRenderTarget::~GL3PlusTextureGpuRenderTarget()
+    {
+        destroyInternalResourcesImpl();
+    }
+    //-----------------------------------------------------------------------------------
     void GL3PlusTextureGpuRenderTarget::createInternalResourcesImpl(void)
     {
         if( mPixelFormat == PFG_NULL )