Remove the "If only executable code is distributed..." paragraph from
the license text. Everything now uses a normal MIT license.
The only code from Khronos that's included in libglvnd is the EGL/GL
header and XML files, which do not contain that paragraph.
Fixes https://gitlab.freedesktop.org/glvnd/libglvnd/-/issues/221
Add a new test, testglxcreatecontext to test the various functions for creating
GLX contexts.
It tests the core GLX functions, the GLX_ARB_create_context and
GLX_EXT_no_config_context extensions, and using a vendor-provided extension
function.
The new test is much simpler than trying to use testglxmakecurrent to test both
context creation and MakeCurrent, especially for trying to test multiple
context creation functions.