graphics/zeno: Unbundle bullet, build with base clang, fix runtime crashes

• Use bullet from devel/bullet instead of the bundled zenustech/bullet3 fork. The fork was built as shared libraries (it forces BUILD_SHARED_LIBS=ON) that libzeno.so linked against but which were never installed, so libzeno.so failed at runtime with libBulletDynamics.so.3.17, libVHACD.so, libBussIK.so, ... not found. Only the parts devel/bullet does not provide (BulletInverseDynamics headers, BussIK, VHACD, URDFImporter) are still taken from the fork and compiled directly into libzeno.so. The single precision flavor is required (zeno assumes btScalar is float), so the build stops early when bullet-double is installed.

• Build with base clang instead of GCC. Mixing libstdc++ with Qt5 and the other dependencies built against libc++/libcxxrt crashed zenoedit in __dynamic_cast as soon as a node was added. The --allow-shlib-undefined -lomp workaround is no longer needed. Added a patch for the only clang build error (PrimUnmerge.cpp).

• Fix LIB_DEPENDS on math/Imath: depend on libImath.so instead of libImath-3_1.so, which no longer exists since Imath 3.2.

• Enable ZenoFX (wrangle nodes, without the CUDA part) and Alembic using graphics/alembic, which was already a dependency but unused. Added graphics/openexr to LIB_DEPENDS.

• Drop the openvdb, assimp, dem-bones-bin and libigl GH_TUPLEs, which were used only by disabled extensions (~73 MB fewer distfiles).

• Bump PORTREVISION.

PR: 299168