Cummulative Sum with Reset Value

Consider the following table:

ID | GROUP_ID | ORDER_VAL | RESET_VAL | VAL 
---+----------+-----------+-----------+-----
1  | 1        | 1         | (null)    | 3   
2  | 1        | 2         | (null)    | 2   
3  | 1        | 3         | (null)    | 1   
4  | 1        | 4         | 4         | 2   
5  | 1        | 5         | (null)    | 1   
6  | 2        | 1         | (null)    | 4   
7  | 2        | 2         | 2         | 3   
8  | 2        | 3         | (null)    | 4   
9  | 2        | 4         | (null)    | 2   
10 | 2        | 5         | (null)    | 2   
11 | 2        | 6         | (null)    | 4   
12 | 2        | 7         | 14        | 2   
13 | 2        | 8         | (null)    | 2   

For each row, I need to compute the cumulative sum of VAL for all previous rows (ordered by ORDER_VAL and grouped by GROUP_ID), but each time a non-NULL RESET_VAL is encountered, I need to use that value for the sum. The rows that follow also need to build on top of the RESET_VAL instead of using the actual sum. Note that each group can have multiple reset values.

This is the result I expect for the above table:

ID | GROUP_ID | ORDER_VAL | RESET_VAL | VAL | CUMSUM
---+----------+-----------+-----------+-----+-------
1  | 1        | 1         | (null)    | 3   | 0
2  | 1        | 2         | (null)    | 2   | 3
3  | 1        | 3         | (null)    | 1   | 5
4  | 1        | 4         | 4         | 2   | 4
5  | 1        | 5         | (null)    | 1   | 6
6  | 2        | 1         | (null)    | 4   | 0
7  | 2        | 2         | 2         | 3   | 2
8  | 2        | 3         | (null)    | 4   | 5
9  | 2        | 4         | (null)    | 2   | 9
10 | 2        | 5         | (null)    | 2   | 11
11 | 2        | 6         | (null)    | 4   | 13
12 | 2        | 7         | 14        | 2   | 14
13 | 2        | 8         | (null)    | 2   | 16

If not for the reset values, I could use a window query:

SELECT temp.*,
       COALESCE(SUM(val) OVER (PARTITION BY group_id ORDER BY order_val ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING),
                0) AS cumsum
FROM temp;

SQLFiddle of the above

I was originally mistakenly thinking that I could just put RESET_VAL at the beginning of the COALESCE, but this won’t work since it won’t reset the value for following rows.

I also tried this solution, but it only resets back to zero, not to a value in the column. Adapting it to do so is proving non-trivial because that value must propagate to all following rows.

A recursive query seems like a natural fit, but I haven’t been able to figure out how to do that as of yet.

I should probably mention that the table I actually have to deal with is much larger (hundreds of thousands to a couple million rows) than the example above, so please mention if there are any performance pitfalls with any answer.