add in High DPI support (aka Retina)

- based on Jørgen's patch with a few bug fixes
Fixed the name of the environment variable to match the name of the hint.
Fixed bug 2100 - directfb fails to build
# User Darren Salt <devspam@moreofthesa.me.uk>
# Date 1379621782 -3600
#      Thu Sep 19 21:16:22 2013 +0100
Work around a false-positive in the X11 mouse wheel code

This false positive occurs when one particular button on my mouse is
pressed. The kernel which I'm using is patched to cause a release event to
be synthesised immediately when the mouse says that this button is pressed
because the mouse doesn't signal release until the button is next pressed.

(Also documents a false negative, observed with the horizontal scroll wheel
on the same mouse.)
Fixed bug 2101 - CWBackPixel causes weird window flickering on window resize

aBothe

I tried to experiment a bit with SDL2 and OpenGL today and noticed that something caused some weird flickering when resizing my nicely drawn SDL2/OpenGL window:
Just after resizing, the background went black and I had to let my OpenGL code redraw the contents..
However, after some hours spent with googling I found out that in OpenGL examples where this CWBackPixel flag was not used when creating X windows, there was no flickering while resizing the window.

See http://www.sbin.org/doc/Xlib/chapt_04.html @ "The Window Background" for more info.