Pass EvaluationContext instead of bContext

2.8x branch added bContext arg in many places,
pass eval-context instead since its not simple to reason about what
what nested functions do when they can access and change almost anything.

Also use const to prevent unexpected modifications.

This fixes crash loading files with shadows,
since off-screen buffers use a NULL context for rendering.

intern/itasc/Scene.cpp

@@ -257,7 +257,7 @@ bool Scene::getConstraintPose(ConstraintSet* constraint, void *_param, KDL::Fram
 	return true;
 }
 
-bool Scene::update(struct EvaluationContext *eval_ctx, double timestamp, double timestep, unsigned int numsubstep, bool reiterate, bool cache, bool interpolate)
+bool Scene::update(const struct EvaluationContext *eval_ctx, double timestamp, double timestep, unsigned int numsubstep, bool reiterate, bool cache, bool interpolate)
 {
 	// we must have valid timestep and timestamp
 	if (timestamp < KDL::epsilon || timestep < 0.0)