Add page about enabling Indirect GLX for Linux and Mac

See this page for details. Instructions for re-enabling it are on this page. For newer Xorg versions, there is also a config flag that is easier to use:

[acaprez@rcf-acaprez xorg.conf.d]$ cat /etc/X11/xorg.conf.d/iglx.conf
Section "ServerFlags"
	Option "IndirectGLX" "on"
EndSection

mentioned in commit c690aad0

mentioned in merge request !113 (merged)

assigned to @acaprez2

closed via merge request !113 (merged)

closed via commit e92e4e9f

closed via commit 770c67e6

mentioned in commit 770c67e6

mentioned in commit e92e4e9f