Allow Negative Z Values for Sprites in the Pipelined Renderer?

[zicklag]

Does it make sense to anybody else to support negative depths for sprites by default? Right now we have a near plane of 0, but I feel like a near plane of -1000 would be useful, so that you have like a "base" depth ( 0 ), and you can put things behind or on top of the base depth. It feels strange to me whenever I make a sprite with a non-positive Z and it disappears.

[alice-i-cecile]

I'd prefer a proper layering solution for 2D (see #1275), but I'm in favor of making this change now.