xf86-video-nested: driver to run Xorg on top of Xorg or something else

= Usage =

This is how I test it:
    X -config my.conf -noreset -retro :1
    (if your Xorg server is old you might need "-sharevts vt7", in case your
    bottom-layer X runs on vt7)

My xorg.conf:

-- begin xorg.conf --
Section "ServerFlags"
    Option "AutoEnableDevices" "false"
    Option "AutoAddDevices" "false"
    Option "AllowEmptyInput" "true"
EndSection

Section "Device"
    Identifier "device1"
    Driver "nested"
    Option "Display" ":0"     # you can omit this
EndSection

Section "Screen"
    Identifier "screen1"
    Device "device1"
    DefaultDepth 24
    Option "Origin" "100 100" # you can omit this
    SubSection "Display"
        Depth 24
        Modes "640x480"
    EndSubSection
EndSection

Section "ServerLayout"
    Identifier "layout1"
    Screen "screen1"
EndSection
-- end xorg.conf --

Mouse and keyboard input events from the client window are forwarded to the nested
xserver, so no mouse/keyboard drivers are needed.

You can also have more than one screen with this driver. Here's an example of a
xorg.conf with 2 screens and a mouse:

-- begin xorg.conf --
Section "ServerFlags"
    Option "AutoEnableDevices" "false"
    Option "AutoAddDevices" "false"
    Option "AllowEmptyInput" "true"
EndSection

Section "Device"
    Identifier "device1"
    Driver "nested"
EndSection

Section "Device"
    Identifier "device2"
    Driver "nested"
EndSection

Section "Screen"
    Identifier "screen1"
    Device "device1"
    DefaultDepth 24
    SubSection "Display"
        Depth 24
        Modes "640x480"
    EndSubSection
EndSection

Section "Screen"
    Identifier "screen2"
    Device "device1"
    DefaultDepth 24
    SubSection "Display"
        Depth 24
        Modes "640x480"
    EndSubSection
EndSection

Section "ServerLayout"
    Identifier "layout1"
    Screen "screen1"
    Screen "screen2" RightOf "screen1"
    InputDevice "mouse1"
EndSection
-- end xorg.conf --