The root cause of the CWE-190 (Integer Overflow or Wraparound) vulnerability in this code is the potential for integer overflow in the calculation of the pointer offset:

```c
nXDst * dstBytesPerPixel + nYDst * nDstStep
```

This calculation is used to determine the starting position of the destination buffer. The vulnerability occurs because:

1. The multiplication and addition operations are performed using unsigned 32-bit integers (UINT32).

2. There are no checks to ensure that the result of these operations doesn't exceed the maximum value that can be stored in a UINT32 (4,294,967,295).

3. If the result overflows, it will wrap around to a smaller value, potentially causing the pointer to point to an incorrect memory location.

For example, if:
- nXDst is very large
- dstBytesPerPixel is greater than 1
- nYDst is also large
- nDstStep is significant

The multiplication and addition could easily exceed the maximum value of UINT32, causing an overflow.

This overflow can lead to various issues:
1. Accessing memory outside the intended buffer.
2. Potential buffer overflows if the resulting pointer is used for writing data.
3. Incorrect data processing if the pointer doesn't point to the intended location.

To mitigate this vulnerability, the code should include checks to prevent integer overflow before performing the calculation, or use a wider integer type (e.g., uint64_t) for the calculation to reduce the risk of overflow.