Math::Geometry: cleaned up Distance test.

Everything is working as expected, which is ahem... unexpected.
Math::Geometry: cleaned up Distance, removed use of Matrix.

The same functionality is now provided in Vector2::cross() which is
also more meaningful.
Use `\bot` instead of `\perp` in math formulas.

`\perp` is for relations, `\bot` is usable as "name". Thanks @JanDupal
for hints :-)
Make Shader::addSource() no-op for empty string.
Make Shader::addSource() move-aware.

Also update addFile() to prevent unnecessary copying.
Shaders: added first version of MeshVisualizer shader.

Rudely written to work only with fairly recent extensions and nothing
less than GLSL 3.30. It requires geometry shaders for wireframe
rendering, without it it behaves the same as Shaders::Flat.
First C++14 TODO :-)