EGL headless visualization

[najlkin]

This PR adds headless (no GUI) visualization using EGL standard. The new EglWindow serves as an alternative backend instead of SdlWindow, which is no longer needed directly and the code uses general GLWindow base class instead. The headless rendering is available in the following ways:

• from command line with option -hl (control key q should be used to quit after the visualization is done)
• saved streams when started with the above option (end of the input stream closes the visualization)
• from scripts, starting them with the above option or through headless command before the visualization command(s), which are then performed without autopausing (end of the script files closes the visualization)
• server started with the -hl option, where end of the input streams also closes the visualization(s)

↪️ dependencies:

• Refactoring with scripts and stream commands help #335

🌟 bonus features:

• moved SDL-based content to sdl directory (and similarly EGL-based to egl), cleaning up file hierarchy
• fixed handling of screenshot command in scripts, as criticized in Screenshot does not behaves reliably in glvs script #254
• fixed caching of MFEM in CI tests (MPI option was not recognized)
• removed using namespace std pollution (i.e., uncontrolled spreading through header files) (cherry-picked in Update C++ standard to 17 #348 )

📎 todo:

• CI building (only Ubuntu)
• server mode
• CI tests?

✔️ testing:

• some cmd lines
• some scripts
• some saved streams
• inline visualization
• js

[tzanio]

Very excited about this 👍

[pazner]

@justinlaughlin, @v-dobrev: Any objections?

[tzanio]

@najlkin, can you please resolve the conflicts?

[najlkin]

Resolved 🫡 🚀

[tzanio]

@GLVis/developers -- please speak up if you have any concerns about this PR which also includes significant refactoring and improvements.

If there are no concerns, we plan to merge this tomorrow (Jan 30th).