SSAO: Use int rounding
Fixed #1313.

In some cases STK was barely playable because of very low refresh rates (25-30Hz).

This may be caused by last changes in display drivers. I remember that previously refresh rates for resolution were in order from the lowest to the greatest. Now I see it in reversed order.
Irrlicht didn't check refresh rates and always set the last available for specified resolution.

Now we use the highest available refresh rate.

Perhaps we should use xrandr library instead of vidmode. It should be much smarter and maybe would solve some multimonitor issues.
Reenable custom light radius
Merge branch 'master' of https://github.com/supertuxkart/stk-code into devel
Allow to run xrandr instead of vidmode as an option
Resolution independent ssao
Use mipmap nearest
Add a 17 tap blur filter

and use it for SSAO
SSAO: Stronger blur + tweak param

Use another algorithm to have gaussian blur that lets us customise
radius more easily.
Remove call to createMeshWelded, now handled in B3D exporter
Avoid displacement related fbo bind in non advanced pipeline
DS buffer is not linear and cant be mipmapped

Use the method provided by SAO paper (ie convert to linear and then
mipmap)
Merge branch 'master' of https://github.com/supertuxkart/stk-code into devel