DS buffer is not linear and cant be mipmapped

Use the method provided by SAO paper (ie convert to linear and then
mipmap)
Light: Import distance properly.

Also remove some silly default param for addlight that could make
argument passing not obvious.
Fix sun light
Replace texture2D by texture in dof.frag
Fix OSX 10.7 compilation
Merge branch 'master' of https://github.com/supertuxkart/stk-code
Fix for #1324
Try to convert icon rendering to new helper function (for GL3 support)
Some tweaking to make ssao more apparent
SSAO: Increase beta constant to avoid error
SSAO: Less abrupt transition
Merge branch 'master' of https://github.com/supertuxkart/stk-code into properties