GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.
We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.
DeformEffect is an abstract class that should be used to write effects
that change the geometry of an actor before submitting it to the GPU.
Just like the ShaderEffect class, DeformEffect renders the actor to
which it has been applied into an FBO; then it creates a mesh and stores
it inside a VBO. Sub-classes can control vertex attributes like
position, texel coordinates and the color.
