3D plots not clipped at axis limits

Simple reproduce script:

``` julia
using PyPlot

# Create a figure with a 3D plot
subplot(111,projection="3d")

# Plot a curve
t = 0:.1:6pi
plot(cos(t),sin(t),t) # nice helix around z axis

# Restrict domain
xlim(0,1)
ylim(0,1)

# Domain for "backdrop is now restricted, but entire helix still drawn!
```

I don't know if this is a problem in `PyPlot.jl` or a more fundamental one in `matplotlib` - for some reason I can't get `matplotlib` working with 3d projections at all when running from plain python. If someone else can get that working, and verify that it's an issue with `matplotlib` rather than `PyPlot`, I'll file the issue there instead.

FWIW, I have `PyPlot` 1.2.9, `matplotlib` 1.3.1, running

```
julia> versioninfo()
Julia Version 0.3.0-rc1+27
Commit 9e5ffd6 (2014-07-15 08:13 UTC)
Platform Info:
System: Linux (x86_64-linux-gnu)
CPU: Intel(R) Xeon(R) CPU E5410 @ 2.33GHz
WORD_SIZE: 64
BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Penryn)
LAPACK: libopenblas
LIBM: libopenlibm
```

Start by running the Julia reproduction script with the reported PyPlot, matplotlib, and Julia versions, then compare the behavior with a plain Python 3D projection if possible. Trace whether axis-limit clipping is handled by PyPlot or matplotlib; done means determining the responsible project and confirming that the helix is clipped to the requested limits.

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