cleanup: style & tips
Fix T42420: Touchpad zoom and scroll gester using touchpad stopped working

Seems to be an own mistake on using Windows API.
CMake: correct own error using undefined var
World background working on cycle nodes.

This is added in the spirit of the general cycles GLSL system
which is pretty much WIP still.

This will only work on cycles at the moment but generating for blender
internal is possible too of course though it will be done in a separate
commit.

This hasn't been tested with all and every node in cycles, but
environment and regular textures with texture coordinates work.

There is some difference between the way cycles treats some coordinates,
which is in world space and the way GLSL treats them, which is in view
space.
We might want to explore and improve this further in the future.

...also </drumroll>
Freestyle: Fix for view map caching not flashed properly in view port rendering.
cmake/windows Change python installation so it does not trigger on simple rebuilds
Fix GLSL drawing of math absolute node.

Patch by Romanov Alexander of blen4web team!

Differential: D842

Thanks for the patch!
Do not use mipmapping on envmaps, can cause discontinuities on
environmental mapping when crossing the seam boundary.

Also output our favourite magenda on missing texture.