perf(filters): optimize IntegralImage, ChannelCombine, CustomConvolution #119

Job	Run time
Build Khronos OpenVX sample	1m 20s
build	1m 39s
Build rustVX (main)	1m 38s
baseline	13s
vision-color	11s
data-objects	13s
image-ops	10s
graph	9s
vision-geometric	37s
KHR extension: user-data-object	7s
vision-features	20s
enhanced-vision (Phase 1 — Min/Max)	13s
vision-statistics	9s
Perf gate (PR vs main)	2m 37s
vision-arithmetic	25s
vision-pyramid	11s
vision-filters	1m 5s
Benchmark & compare (rustVX vs Khronos sample)	4m 41s
	15m 58s

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