pex plugin: Fix radial emitter

The radial emitter was lacking one assignment present in the gravity emitter,
with the result that it didn't display anything.
pex plugin: Cleanup and bug fixes

This looks larger than it is.

There was a ton of duplicated code between the gravity and radial script
cases, and that's simplified and cleaned up. Also, there's some overly
complicated computations, and dead code. It appears that the intended
angle computations for gravity emitters weren't quite correct. The
sourcePosition and sourcePositionVariance values were ignored. The code
to read in attributes could easily overrun buffers. In theory, all of
these should be fixed now.
particles-pex-plugin sample: Clean up .pex file, update usage

The sourcePosition and sourcePositionVariance values were being mostly
ignored, and the usage of them with setRect was an extra complication.
Instead, we now handle those internally in the plugin code. Update
sample accordingly.

Also clean up the sample so it's more readable.
Fix miscapitalized attribute name.

There was a typo in one of the attribute names. We still accept the
old attribute name, but we now recognize the correct capitalization
and update the example to match it.
Merge pull request #939 from seebs/seebs/pexpluginfixes

Seebs/pexpluginfixes