Add gs_device_type function

This allows a programatic way of determining the type of graphics module
currently active.

libobs/graphics/device-exports.h

@@ -24,6 +24,7 @@ extern "C" {
 #endif
 
 EXPORT const char *device_name(void);
+EXPORT int device_type(void);
 EXPORT const char *device_preprocessor_name(void);
 EXPORT device_t device_create(struct gs_init_data *data);
 EXPORT void device_destroy(device_t device);