Relax lifetime restrictions on as_raw_buffer, as_raw_nopadding_buffer, and save_as_image for windows_capture::frame::FrameBuffer

[winestone]

We probably intend these lifetimes to look like:

 impl<'a> FrameBuffer<'a> {
-    pub fn as_raw_buffer(&'a mut self) -> &'a mut [u8] {
+    pub fn as_raw_buffer<'b>(&'b mut self) -> &'b mut [u8] {
         self.raw_buffer
     }

which is equivalent to letting the lifetimes be inferred I think.

Hope this PR helps, let me know if there are any changes you'd like, thanks!

---

For some context, this was preventing me from writing a function like:

    fn from_frame_buffer<'a, 'fb: 'a>(frame_buffer: &'a mut FrameBuffer<'fb>) -> Self {
        let buffer = frame_buffer.as_raw_buffer().to_vec();
        let width = frame_buffer.width();

and I would get an error like:

error[E0502]: cannot borrow `*frame_buffer` as immutable because it is also borrowed as mutable
  --> src/main.rs:45:26
   |
43 |     fn from_frame_buffer<'a, 'fb: 'a>(frame_buffer: &'a mut FrameBuffer<'fb>) -> Self {
   |                              --- lifetime `'fb` defined here
44 |         let buffer: Vec<u8> = frame_buffer.as_raw_buffer().to_vec();
   |                               ----------------------------
   |                               |
   |                               mutable borrow occurs here
   |                               argument requires that `*frame_buffer` is borrowed for `'fb`
45 |         let width: u32 = frame_buffer.width();
   |                          ^^^^^^^^^^^^ immutable borrow occurs here

Essentially when I call frame_buffer.as_raw_buffer(), it tries to borrow it for the lifetime of 'fb above, which is probably not what we want (I just wanna borrow it just for this statement, not for the whole 'fb lifetime).

[NiiightmareXD]

Nice, thanks.