samples: add OpenVX graph pipelining CPU+GPU sample

[simonCatBot]

Summary

Adds a new C++ sample under samples/c_samples/pipelining that demonstrates the OpenVX vx_khr_pipelining extension on a mixed CPU+GPU workload.

The sample runs the same vision graph in two modes:

• --pipeline 0 — synchronous vxProcessGraph loop (baseline).
• --pipeline 1 — QUEUE_AUTO pipelined enqueue/dequeue with multiple input/output buffers.

Graph: RGB -> ColorConvert -> ChannelExtract(Y) -> Box3x3 -> U8

Both paths compute an identical per-frame checksum aggregate, and the pipelined path reports a higher fps so the benefit is immediately visible.

What was added

• samples/c_samples/pipelining/pipelining.cpp — the sample application.
• samples/c_samples/pipelining/CMakeLists.txt — build file matching the existing canny sample.
• Updated samples/c_samples/README.md with build/run instructions.

Verification performed

Built and tested locally on a Radeon gfx1151 GPU and on the CPU-only backend:

Mode	Backend	fps (60 frames @ 640x480)	checksum aggregate
sync	HIP	~447	2309715656
pipe	HIP	~503	2309715656
sync	CPU	~352	2304694854
pipe	CPU	~531	2304694854

Both backends pass the checksum-equality test and show measurable speed-up when pipelining is enabled.

Checklist

• Builds on HIP backend.
• Builds on CPU-only backend.
• --pipeline 0 and --pipeline 1 produce the same aggregate checksum.
• --pipeline 1 reports higher fps than --pipeline 0.
• Does not touch any existing library code; only adds a new sample.

[daniellegillai]

Tested on Radeon RX 9070 XT / Ryzen 9 9950X 16-Core Processor, using TheRock Nightly 10.1.0 2026-08-05:

Mode	Backend	fps (60 frames @ 640x480)	checksum aggregate
sync	HIP	~422	2309715656
pipe	HIP	~456	2309715656
sync	CPU	~450	2304694854
pipe	CPU	~498	2304694854

Both backends pass the checksum-equality test and show speed-up when pipelining is enabled. One note is that CPU outperformed HIP backend, @kiritigowda is that reasonable?