Implement correctly blended premultiplied alpha

[emilianavt]

This PR implements the new blending option for premultiplied alpha described in issue #38. It renames the old option to "Converted Premultiplied Alpha (legacy)" and adds a new option "Premultiplied Alpha". I wasn't able to update the Chinese string table, so that still needs to be done.

Here's an example of the difference. Premultiplied alpha source sent from Unity:

Converted premultiplied alpha gets desaturated under certain circumstances, with part of the yellow particles turning white:

Using the correct blend settings, it is applied without desaturation:

[campbellwmorgan]

Thanks very much for this! We'll send this through QA to test it out early next week and let you know, but looks very promising. Thanks again