Blender Git Loki

More F-Modifier Tweaks:

This commit started out aiming to make the "Stepped" F-Modifier (committed last night) even more useful, but ended up fixing a few other finer-points of how F-Modifiers work.

Firstly, the new stuff:
I've addded options to the Stepped F-Modifier to not affect frames before and/or after specified values, and renamed the 'start offset' to 'offset' for clarity.

The main objective of this is to allow Stepped F-Modifiers to only affect certain time ranges, so that by layering/using multiple instances of the F-Modifier, it can be possible to have multiple stepping-sizes.

This allows for effects like:
http://www.pasteall.org/blend/2230
or in words, it provides a convenient mechanism for animators to specify whether sections of the animation is shown "on twos", "fours", or even "forty-second-ths plus a smidgen", as can be easily done with 2D.

Assorted changes to support this:
* Properly fixed up how F-Modifiers that work with time, evaluate the time to evaluate the curve at. Now layered time effects like this should be possible in a much nicer way.

* Added proper value range validation/clamping to many properties. There are still a lot more that need checking, but at least more properties now do "the right thing".