Clarify optimizeimg docs; add warnings

The validator will now warn if it detects that a crushed output
is fed into something that is not a crusher.

The is_crusher method of an optimizer shall return True if the
optimisation process is lossless, and does try to find optimal
encoding parameters as opposed to only removing unneeded channels
or reducing palettes.

overviewer_core/settingsValidators.py

@@ -160,16 +160,25 @@ def validateBGColor(color):
 def validateOptImg(optimizers):
     if isinstance(optimizers, (int, long)):
         from optimizeimages import pngcrush
-        logging.warning("You're using a deprecated definition of optimizeimg. We'll do what you say for now, but please fix this as soon as possible.")
+        logging.warning("You're using a deprecated definition of optimizeimg. "\
+                        "We'll do what you say for now, but please fix this as soon as possible.")
         optimizers = [pngcrush()]
     if not isinstance(optimizers, list):
-        raise ValidationException("optimizeimg is not a list. Make sure you specify them like [foo()], with square brackets.")
-    for opt in optimizers:
+        raise ValidationException("What you passed to optimizeimg is not a list. "\
+                                  "Make sure you specify them like [foo()], with square brackets.")
+
+    for opt, next_opt in zip(optimizers, optimizers[1:]) + [(optimizers[-1], None)]:
         if not isinstance(opt, Optimizer):
             raise ValidationException("Invalid Optimizer!")
 
         opt.check_availability()
 
+        # Check whether the chaining is somewhat sane
+        if next_opt:
+            if opt.is_crusher() and not next_opt.is_crusher():
+                logging.warning("You're feeding a crushed output into an optimizer that does not crush. "\
+                                "This is most likely pointless, and wastes time.")
+
     return optimizers
 
 def validateTexturePath(path):