feat(ugc): ray_backend=hiprt traces on the GPU with HIPRT and Orochi (optional build)

HIPRT (MIT) behind the CMake option DLU_HIPRT (off): its headers come from its
SDK (HIPRT_ROOT, else ROCm's /opt/rocm) and are copied next to the servers; its
library is loaded when first used (hiprtew), as HIP or CUDA are by Orochi
(MIT, fetched pinned by hash; CUDA when its toolkit is found). The trace
kernels (nearest hit skipping the triangle a ray leaves, any hit) are compiled
the first time and kept in cache/hiprt. One GPU context for the process
(hiprt_device picks the GPU); the workers take turns on it. When HIPRT, the
GPU or a scene's upload fails, Embree is used instead, and the UGC server logs
why at start.

For a GPU the rays go in batches (UgcRays::Scene gets batch queries; the CPU
backends answer them a ray at a time):
- hidden faces: with a batch backend the paths are traced side by side, a
  bounce at a time (the path code split into Start, Scatter and Bounce, the one
  by one tracing unchanged); the same paths with the same random numbers, so
  the same triangles are decided (tested with builtin side by side)
- the occlusion bake and the denoised icons' traced occlusion always ask in
  batches (the same rays, the same results)

Its symbols are hidden: the servers export theirs (-rdynamic), and HIPRT's
library, which has an Orochi of its own, would otherwise call ours.

Check: configure with -DDLU_HIPRT=ON on a machine with ROCm (or HIPRT's SDK)
and an AMD RDNA or NVIDIA GPU; UgcServer --make-model x.lxfml out hiprt; the
UGC tests (hits, hidden faces and occlusion against builtin); a build without
it leaves everything as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

resources/ugcconfig.ini

@@ -146,8 +146,10 @@ hsr_method=toolbox
 hsr_fast_resolution=1024
 
 # What traces the rays of the hidden faces' paths and of the occlusion: builtin, embree (Intel Embree on the CPU) or
-# hiprt (the GPU, when built with DLU_HIPRT and the machine has one; else embree).
+# hiprt (the GPU, when built with DLU_HIPRT and the machine has one; else embree). hiprt_device: which GPU (0: the first
+# HIP or CUDA device; restart to change).
 ray_backend=builtin
+hiprt_device=0
 
 # denoise: off, or oidn (when built with DLU_OIDN; else off): a model's icon is drawn from its colors before the
 # occlusion bake with its occlusion traced per pixel (denoise_samples rays from each pixel of the supersampled image)