Split AppData::m_appData into platform (as-is: m_appData) and rendere…

…r depended (m_rendererData introduced)

Premises: https://github.com/ocornut/imgui/blob/6b2e03c5b1bc4f99dbd86d10e5fd12af9e3fe1c2/imgui.h#L2005-L2009

This pull-req addresses: john-chapman#63

im3d.h

@@ -549,6 +549,7 @@ struct AppData
 	float  m_snapScale                       = 0.0f;                    // Snap value for scale gizmos. 0 = disabled.
 	bool   m_flipGizmoWhenBehind             = true;                    // Flip gizmo axes when viewed from behind.
 	void*  m_appData                         = nullptr;                 // App-specific data.
+	void*  m_rendererData                    = nullptr;                 // Renderer-specific data.
 
 	DrawPrimitivesCallback* drawCallback     = nullptr; // e.g. void Im3d_Draw(const DrawList& _drawList)