Simplify creation of OpenGL contexts under X11 platform (#1005)

The previous approach FLTK used to create OpenGL contexts under the X11 platform
followed a different code path to create contexts for OpenGL version 3 and above
than to create contexts for OpenGL versions 1 or 2.
The FLTK code followed exactly "Tutorial: OpenGL 3.0 Context Creation (GLX)"
of the official OpenGL wiki, see:
https://www.khronos.org/opengl/wiki/Tutorial:_OpenGL_3.0_Context_Creation_(GLX)
That code worked OK with Debian 11 and with any tested Linux configurations
in a VM running on Apple hardware. However, it failed with Debian 12 and later
on native Linux boxes to create GL3 contexts with the FL_DOUBLE flag.
That's what issue #1005 reports.

A first attempt to fix #1005 followed instructions given at:
https://registry.khronos.org/OpenGL-Refpages/gl2.1/xhtml/glXIntro.xml
which supposedly describe how to create OpenGL contexts with GLX.
That had no effect on issue #1005.

This commit erases all attempts to use OpenGL3-specific calls or even
the more modern glXCreateNewContext() function that appears with
GLX version 1.3. The committed code uses only OpenGL 1-age
functions to create OpenGL contexts for X11 windows.
Created contexts follow the "Compatibility Profile" which means they
are compatible with both modern OpenGL3-style and legacy OpenGL1/2-style.

This appears to fix issue #1005.

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