The root cause of the CWE-787 vulnerability in the given code slice is a potential buffer overflow due to an incorrect calculation of the remaining buffer space.

The vulnerability occurs at line 35, where the code checks if there is enough space left in the buffer to write 2 pixels (width + 3 is likely a typo and should be width + 2, as each pixel is 2 bytes). However, this check is insufficient because it does not account for the fact that the buffer pointer `frame` has already been incremented by `v` bytes in the previous line (line 33).

The correct check should be `if (frame_end - frame < width + 2 + v)`, which takes into account the remaining buffer space after the previous increment.

The incorrect check can lead to a buffer overflow when the remaining buffer space is less than `width + 2` bytes, causing the program to write beyond the bounds of the buffer. This can result in undefined behavior, including crashes, data corruption, or potentially even code execution.

To fix the vulnerability, the check at line 35 should be modified to correctly account for the remaining buffer space. Additionally, the code should ensure that the buffer pointer `frame` is not incremented beyond the bounds of the buffer.

Here is the corrected code:
```c
if (frame_end - frame < width + 2 + v) {
    // handle buffer overflow error
}
```
Alternatively, the code can be rewritten to avoid the need for this check altogether by using a safer approach to buffer management.