vo_gpu: refactor HDR peak detection algorithm

The major changes are as follows:

1. Use `uint32_t` instead of `unsigned int` for the SSBO size
   calculation. This doesn't really matter, since a too-big buffer will
   still work just fine, but since `uint` is a 32-bit integer by
   definition this is the correct way to do it.

2. Pre-divide the frame_sum by the num_wg immediately at the end of a
   frame. This change was made to prevent overflow. At 4K screen size,
   this code is currently already very at risk of overflow, especially
   once I started playing with longer averaging sizes. Pre-dividing this
   out makes it just about fit into 32-bit even for worst-case PQ
   content. (It's technically also faster and easier this way, so I
   should have done it to begin with). Rename `frame_sum` to `frame_avg`
   to clearly signal the change in semantics.

3. Implement a scene transition detection algorithm. This basically
   compares the current frame's average brightness against the
   (averaged) value of the past frames. If it exceeds a threshold, which
   I experimentally configured, we reset the peak detection SSBO's state
   immediately - so that it just contains the current frame. This
   prevents annoying "eye adaptation"-like effects on scene transitions.

4. As a result of the previous change, we can now use a much larger
   buffer size by default, which results in a more stable and less
   flickery result. I experimented with values between 20 and 256 and
   settled on the new value of 64. (I also switched to a power-of-2
   array size, because I like powers of two)

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