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).