Fix linux compilation (no nullptr).
Temporary fixed windows compilation (glGetStringi not known). Needs
a proper fix!!
Merge branch 'master' of github.com:supertuxkart/stk-code
Fixed height computation. Moved adding of gravity center
 shift, which probably fixed the problems we had with
 'wheels inside kart chassis' tests earlier.
Set the graphical suspension depending on physical suspension (capped).
Adjusted graphical chassis to be 0.1 higher to have similar look at previously
and avoid graphical chassis going into terrain,
xrandr: make sure that output_id and old_mode are declared
xrandr: don't restore resolution if output was disconnected during game
xrandr: move restore resolution to separated function because it makes a mess...
Use singleton template for VAOManager (ex VBOGatherer)
Forgot to free a temp array.
Properly clean program object in load shader function.
Use stream_copy for tfb buffers.
Add a function that kills shaders and use it when reloading shader.

Thus there is no mesh corruption.
Fix leak
Fix hasGLExtension on Windows.

Windows requires loading the glGetStringi function explicitly.
Also included fallback for when it is not found.
Fixed shadows with a hack.
Removed artificial Y offset to allow evaluation of more karts.
Make the graphical y offset configurable (and basically disable
it for now by setting it to 0).
Fix suspension to some more realistic values.
Use sqrt(speed) to scale the collision impulse, which results in
weaker impulses when hitting the terrain.