Forcing active layers

When implementing or debugging layers stuff it is often useful to force a layer to be active - active layers have different behaviour from inactive ones, e.g., only active layers can have mask layers - and it is nice to know whether a glitch is due to code on the active or inactive path, or because a layer should be active and isn't, or vice versa. The easiest way to force a single layer to be active is to give it a 1 pixel 3d transform (this is ultra-hackey, please don't use this except for debugging). But sometimes it is nice to make all layers active, or be able to switch between active/inactive without editing HTML. So, I added a pref to do just that, in about:config, set layers.force-active to true and all possible layers should become active. This shouldn't affect which content gets a layer, if it didn't get its own layer before it won't get one with the pref. This is not meant for normal browsing and may cause all kinds of bugs.