Fixed prefetching logic

[tiif]

This PR fixed the prefetching logic when the playing is not sequential.

New prefetching algorithm:

• Constraint: The frames within the buffer are required to form a contiguous sequence, meaning that consecutive frame indices must be present without any gaps.

• When the requested frame is not inside the prefetching buffer, it will discard all previous frames, and start to prefetch from current frames.

• For example, if the current prefetch buffer contains these frames:

  [1, 2, 3, 4]

  Then we request for the 10th frame, the prefetch buffer state will become:

  [11, 12, 13, 14]

---

Test:

• Setting:

  • buffer capacity= 4
  • make sure there is > 24 ply ascii files in ./Ply
  • Command used: cargo run --bin vvplay_async -- --buffer-capacity=4 ./Ply

• Test 1: Request for a ready frame inside the buffer

  
  // buffer setup
  [1(ready), 2(ready), 3(ready), 4(ready)]

//then use UI to jump to frame 3

//result [3(ready), 4(ready), 5(ready), 6(ready)]

- Test 2: Request for a not ready frame inside the buffer

// buffer setup [1(ready), 2(ready), 3(ready), 4(decoding)]

//use UI to jump to frame 4

//buffer result [4(ready), 5(decoding), 6(decoding), 7(decoding)]

- Test 3: Request for a frame which is not inside the buffer > current frame range in buffer, and is not in the playback cache either

// buffer setup [1(ready), 2(decoding), 3(decoding), 4(decoding)]

//use UI to jump to frame 10

// result [10(decoding), 11(decoding), 12(decoding), 13(decoding)]

- Test 4: Request for a frame which is not inside the buffer < current frame range in buffer, and is not in the playback cache either

// buffer setup [20(ready), 21(ready), 22(ready), 23(ready)]

// use UI to jump to frame 8

// buffer result [8(decoding), 9(decoding), 10(decoding), 11(decoding)]

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### Extra note:
- If the ``CameraPosition`` of previous frame is retrievable, the program will use that; else, it will just use the default ``CameraPosition``
- Once a frame is requested, there is no way to cancel it. All the previous ``FrameRequest`` needs to be processed in order for the current ``FrameRequest`` to be handled.