Depth calculation

[sellendk]

Currently, there is an assumption in various shaders throughout megamol regarding the calculation of the fragment depth value.

For example, in the ellipsoid shader of the GlyphRenderer

depth_out = ((depth / depth_w) + 1.0) * 0.5;
// assumes gl_DepthRange.near = 0.0 and gl_DepthRange.far = 1.0

If these values in the host code are changed by glDepthRange(), the DB will be wrong.
A possible solution to account for a possible change of the these values could be

float near = gl_DepthRange.near;
float far = gl_DepthRange.far;
depth_out = ((far - near) / 2.0) * (depth / depth_w) + (far + near) / 2.0;
// same as depth_out = ((depth / depth_w) + 1.0) * 0.5
// if near = 0.0 and far = 1.0