Make all constructors explicit by default.
Use threaded shader compilation everywhere.
bullet: prefix member variables with underscore.
cubemap: prefix member variables with underscore.
primitives: prefix member variables with underscore.
text: prefix member variables with underscore.
textured-triangle: prefix member variables with underscore.
triangle: prefix member variables with underscore.
viewer: prefix member variables with underscore.
Adapted to Magnum changes.
text: minor cleanup