Add gs_device_name function

This returns the name of the device, "Direct3D 11" or "OpenGL"
respectively.

libobs-opengl/gl-subsystem.c

@@ -182,6 +182,11 @@ void convert_sampler_info(struct gs_sampler_state *sampler,
 	               info->max_anisotropy, sampler->max_anisotropy);
 }
 
+const char *device_name(void)
+{
+	return "OpenGL";
+}
+
 const char *device_preprocessor_name(void)
 {
 	return "_OPENGL";