Blender Git Loki

Bugfix #4535

A proper check for the error "No Camera" on rendering can only be made
after a renderwindow was initialized. The error menu then shows in the
render window, which appears to not work OK in windows ATI (again!).

So; I've moved the test to before the renderwindow is activated, this is
not a test checking on the entire render pipeline (like Composite nodes or
sequence strips). In case more complex setups cannot render, an error is
printed in the console only.