Doc++
Finally implemented parallelizable shader compilation and linking.

As g_truc said long ago:
https://twitter.com/g_truc/status/352778836657700866

Currently there is not much use of this as the stock shaders are
compiled one by one (and doing it differently would make things
needlessly overcomplicated), but the users can do parallel compilation
of their own shaders.

Also removed a bunch of now-unneeded TODOs and made the linker/compiler
code nearly similar. Also the whole Shader::compile() call now does two
allocations in total instead of two allocations for each shader.
Print non-error shader compilation/linking messages to Warning output.

Instead of schizophrenic Debug/Error.
Fixed, cleaned up and updated AbstractShaderProgram documentation.
Shaders: making use of parallel compilation.

Compiling fragment and vertex shader simultaenously, at least. Nothing
more can be done for now.

Also removed weird duplicate compile/link calls from MeshVisualizer,
went unnoticed since b9a72bd3d1fd2bf763ca43a8cde7726583b671c2. Why did I
do that?!
Fix ShaderGLTest on ES3.