[vtkusers] EGL offscreen rendering

[Utkarsh Ayachit]

Elhassan,

For VTK 8.1, you can set the following CMake variables:

VTK_USE_X=OFF
VTK_OPENGL_HAS_EGL=ON
EGL_INCLUDE_DIR: Path to directory containing GL/egl.h.
EGL_LIBRARY: Path to libEGL.so.
EGL_opengl_LIBRARY: Path to libOpenGL.so.

> How can I visually debug the output of the EGL rendering?

Currently, we don't support an on-screen rendering with EGL.

Utkarsh