[magnesj]

Adds a --headless command line option that loads a project and exports 3D view and plot snapshots without showing any windows, so batch runs and regression tests can run on machines without a display or OpenGL hardware.

Fixes #15098

Changes

• --headless renders 3D views with software OpenGL (Mesa llvmpipe, opengl32sw.dll on Windows) and keeps all windows and progress dialogs hidden.
• Log messages are written to stdout. On Windows the process attaches to the parent console, so logs show in PowerShell and cmd.
• Snapshot export returns errors, and a failed export gives a non-zero exit code.
• Hidden viewers and plot windows get deferred resize events, fonts, layout and overlay frames, so snapshots match the requested size and content.
• Snapshot export failures are reported through the Python API.
• The Linux launcher script supports headless runs (also related to Octave Dialog Box Functions Not Fully Working #1015).
• Adds a Python example and a class diagram.

Usage

ResInsight.exe --headless --project model.rsp --savesnapshots all --snapshotfolder out
ResInsight.exe --headless --regressiontest <regression-test-folder>

Testing

• The ProjectFiles/2026 regression tests pass in headless mode with exit code 0.
• 3D views and most plots differ from the GUI baselines by 0.1-1.7% of pixels.
• The logarithmic histogram plots differ by about 8.5%, because page margins are not included when rendering hidden multi plots.