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/graphics/graphics-imports.c

@@ -115,6 +115,7 @@ bool load_graphics_imports(struct gs_exports *exports, void *module,
 	GRAPHICS_IMPORT(texture_map);
 	GRAPHICS_IMPORT(texture_unmap);
 	GRAPHICS_IMPORT_OPTIONAL(texture_isrect);
+	GRAPHICS_IMPORT(texture_getobj);
 
 	GRAPHICS_IMPORT(cubetexture_destroy);
 	GRAPHICS_IMPORT(cubetexture_getsize);