Add dummy GL texture flag & direct object access

- Add dummy GL texture support to allow libobs texture references to be
   created for GL without

 - Add a texture_getobj function to allow the retrieval of the
   context-specific object, such as the D3D texture pointer, or the
   OpenGL texture object handle.

 - Also cleaned up the export stuff.  I realized it was all totally
   superfluous.  Kind of a dumb moment, but nice to clean it up
   regardless.

libobs/util/windows/ComPtr.hpp

@@ -75,6 +75,8 @@ public:
 	inline T **Assign()                {Clear(); return &ptr;}
 	inline void Set(T *p)              {Kill(); ptr = p;}
 
+	inline T *Get() const              {return ptr;}
+
 	inline    operator T*() const      {return ptr;}
 	inline T *operator->() const       {return ptr;}