Transitions--changes to fade screen

[alan eborn]

 * The "fade"-screen is now by default simply a semi-transparent DirectFrame, instead of a model loaded automatically.
        - The Panda-provided model can still be used by loading it manually.

 * Changed uses of "setColor" and "lerpColorInterval" to "setColorScale" and "lerpColorScaleInterval", respectively.
        - This means that the changes that Transitions makes only tints fade-models, rather than recolouring them as was previously the case. For one, I found that the use of "setColor" was interfering with the "default model" when that model was loaded manually.

 * A side-effect is that the Panda-provided fade model is now more transparent that it previously was: the model file specifies a degree of transparency (fifty percent, I think it was), which was, I believe, being overwritten by the calls to "setColor". Using "setColorScale", both the model's native transparency and the transparency being specified by the method that shows the fade-screen are being multplied together. However, this is what I would expect to have happen, so I'm inclined to leave it in.

 * In "setFadeModel", added "self.fade.destroy()" before "del self.fade", as I found that it didn't seem to be properly cleaned up without the call.

Please see the attachment for the patch.

[rdb (rdb)]

I don't really understand why it's necessary to change the semantics when there doesn't appear to be a good reason to. If the aim of this change is to replace the fade model with a frame, then it seems just unnecessary to use setColorScale.

I checked in the other changes:
https://github.com/panda3d/panda3d/commit/e33cac03fe59a11d7578cd15c1a3599f4003db0f

Thank you! It feels good not to have unnecessary dependencies on loaded models.