preview: QtPreview: Copy source to a temporary line buffer for speed #471
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This simple change reduces viewfinder CPU-per-frame by a factor of at least 2 (tested on both ARM32 and ARM64).
Though where preview rate was limited by CPU it might not actually reduce total cycles (but might improve frame rate).
This reduces the benefit of the previous "U,V sharing" optimization, but the latter has been retained, to reduce code churn.
Further optimizations such as vectorizing the YUV->RGB conversion in signed 16-bit (rather than float) would yield diminishing returns and are not planned. If we really wanted further speedup we should arrange for a correctly-sized RGB888 source image!