Address issues with `openglsupport` test.

The existing test fails on several systems due to GL runtime version mismatches,
the use of deprecated features, and memory errors due to improper use of GLUT.
The test was modified to:

- Run within a display function, allowing proper GLUT cleanup.
- Generate dynamic shaders with a supported GLSL version string and output variables.
- Report shader compilation errors.
- Check GL context version before launching version-specific tests.

Note that most of the existing `OpenGLSupport` module and tests rely on deprecated
features (e.g. fixed-function pipeline). The test was modified to allow it to
pass on various systems. We might want to consider removing the module or re-writing
it entirely to support modern OpenGL.  This is beyond the scope of this patch.

Testing of legacy GL (for platforms that support it) can be enabled by defining
`EIGEN_LEGACY_OPENGL`.  Otherwise, the test will try to create a modern context.

Tested on
- MacBook Air (2019), macOS Catalina 10.15.7 (OpenGL 2.1, 4.1)
- Debian 10.6, NVidia Quadro K1200 (OpenGL 3.1, 3.3)

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