Implement variance shadow mapping in the fragment shader.
Implement variance shadow mapping in the fragment shader.

We use Chebyshev inequality. For a given fragment at depth z,
P(fragment in shadow) ≤ (variance / (variance + (z - mean)^2)). We use
this upper bound as a shadowing factor.


Former-commit-id: 78339cde802f41a9b2f8ffe3b79aa929385f98a8
kosua20/GL_Template,kosua20/GL_Template