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-d3d11/d3d11-subsystem.hpp

@@ -30,6 +30,7 @@
 #include <util/base.h>
 #include <graphics/matrix4.h>
 #include <graphics/graphics.h>
+#include <graphics/device-exports.h>
 #include <util/windows/ComPtr.hpp>
 #include <util/windows/HRError.hpp>
 
@@ -37,8 +38,6 @@ struct shader_var;
 struct shader_sampler;
 struct gs_vertex_shader;
 
-#include "d3d11-exports.h"
-
 using namespace std;
 
 /*