fix(linux): POSIX sh launch wrappers for all executables; load GL via system GLEW

[mcoliver]

Fixes #66. Fixes #68.

Linux builds couldn't run their own tools: the launch wrappers were lost in the rv → utv rename, and binaries linked OpenGL extension functions that Mesa's libGL doesn't export. This PR fixes both, and Linux CI now runs the full test suite against the result.

#66: Linux launch wrappers

• rv_stage now wraps every Linux executable (renaming the binary to <name>.bin). It uses the target's own <target>.wrapper if present, otherwise the new generic src/bin/linux_wrapper.sh. Previously the <target>.wrapper lookup no longer matched the renamed utv* targets, so utvio, utvls, utvpkg, utvpush, utvshell, utvprof and utv had no wrapper.
• The wrappers are POSIX sh (no tcsh dependency). They resolve symlinks (rv, rvio, …) with readlink -f and exec <name>.bin.
• Library path: UTV's lib/, the bundled OpenSSL (unless RV_USE_SYSTEM_OPENSSL), and the dependency prefix (UTV_DEPS_ROOT, else Homebrew) using each formula's own opt/<formula>/lib[64].
• The GL driver always comes from the system:
  • Homebrew's mesa, libglvnd, libdrm, llvm*, libpciaccess and libxshmfence are skipped.
  • The system libGL.so.1/libEGL.so.1 (found via ldconfig) are preloaded.
  • Without this, Homebrew's software Mesa, pulled in as a dependency of glew, replaced the system/GPU driver through library RUNPATHs.
  • UTV_NO_SYSTEM_GL_PRELOAD=1 disables the preload.
• The main app keeps its extras (-d/--debug via gdb, padsp, unsetting QT_PLUGIN_PATH/LANG) in src/bin/apps/rv/utv.wrapper. The 16 per-tool tcsh wrappers are removed.

#68: OpenGL on Mesa

• TWK_USE_GLEW on Linux: GL entry points are loaded through the system GLEW (shared libGLEW, one function table for all modules) instead of being linked directly (GL_GLEXT_PROTOTYPES), as Windows does with its bundled GLEW.
• TWK_GLEW_INIT(proc) in TwkGLF/GL.h: glewInit(proc) for the bundled Windows GLEW. On Linux it calls glewInit() and treats GLEW_ERROR_NO_GLX_DISPLAY as success, so it works with GLEW 2.2 (distros) and 2.3 (Homebrew), under GLX and EGL/Wayland.
• utvio's offscreen GLX device (FBOVideoDevice):
  • initializes GLEW;
  • sets CWBorderPixel (X rejects a window whose visual depth differs from the root's without it);
  • reports missing display/visual/context clearly;
  • installs an X error handler that logs and continues. The renderer issues a glXMakeCurrent from a second thread; GLX reports BadAccess, and Xlib's default handler was exiting the process.
• Include order: GLEW is included before gl.h in the Linux paths that mix Qt/Python headers and GL (MuUICommands.cpp, PyUICommands.cpp, …).
• OSMesa: TwkGLFMesa compiles with TWK_USE_MESA, which keeps GLEW out of it.

CI

• The Linux ctest step no longer sets LD_LIBRARY_PATH (the wrappers do it) and no longer excludes LoadingSharedLibrariesTest or image-format-roundtrip. utvio is no longer an expected failure in cli-smoke.
• It also runs one utvio render under Xvfb and prints the output, as a debugging aid.
• Result: Linux 178/178 tests pass, including all 58 LoadingSharedLibrariesTest cases, cli-smoke, and the image format round-trip. macOS and Windows are green.

Known issues left open

• fix(linux): IPCore ApplicationTest and AudioRendererTest crash (SIGSEGV) #78: ApplicationTest / AudioRendererTest crash in their test bodies (now that they load); excluded on Linux CI.
• fix: FITS files written by utvio don't read back (Windows: wrong size, Linux: crash) #67: FITS read-back segfaults on Linux; marked as an expected failure.
• rvio_sw (OSMesa build of rvio) still builds, but is probably broken at runtime: it relies on handing OSMesa's proc lookup to GLEW, which the system GLEW can't do. OSMesa was removed from Mesa 25.1. utvio under Xvfb/llvmpipe covers headless rendering. Decision pending on dropping it.
• The stray cross-thread glXMakeCurrent now logs a warning instead of exiting. It's harmless, but worth cleaning up in the renderer.

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