Removed unneeded member variables, removed wrong assertions and wrong
documentation (most of the state they were fobidding is actually valid).
Retrieving shader log with full length, properly printing non-error
messages to debug output.
Each shader must now be compiled explicitly using compile(), which is
slightly better for the user as it is possible to check compile status
instead of having it weirdly hidden inside attachShader(). link() now
also returns linking status.
The methods return reference instead of pointer, as the class is
commonly created on the stack. Removed static functions
Shader::fromFile() and Shader::fromData(), as they are not needed now.
Also asserting that the file exists and is readable in addFile().
* In shader uniforms (projectionMatrix makes more sense than projection
alone)
* For underlying types for SceneGraph transformation. It is already
used in Drawable::clean() as transformationMatrix, so why not use it
also in AbstractFeature::clean(). Moreover, clean() could be in
future also done using something else, this helps to distinguish the
type just from parameter name.
* In Physics shapes - applyTransformationMatrix() (as it could be in
future also done using something else).