Added AbstractShaderProgram::attachShaders().

Convenience overload to attachShader(), allowing the user to specify
more than one shader at once. Just a complement to initializer-list
versions of compile() and link(), but without any performance
difference.
Shaders: using attachShaders() everywhere.
TextureTools: use threaded shader compilation also here.
Added Renderer::enable()/disable().

Should be preferred to Renderer::setFeature() to avoid unnecessary
branching. Also a bit shorter to write.
Use threaded shader compilation etc. also in tests.