Rename `ext::Blur` to `ext::CentralLimitBoxBlur` and rework ex 25

[devshgraphicsprogramming]

Description

Upgrade the example so it matches the new APIs

Solution proposal

Use shared memory and nbl_glsl_workgroupInclusiveAdd to have fast and easy Prefix Sums of entire rows/columns.

Do multiple box blurs in single dispatch.

Follow the style of examples of ext::FFT and ext::Tonemapper.

Additional context

Old discussions https://github.com/buildaworldnet/IrrlichtBAW/issues/70