GPU particle example with alpha

[micuat]

It's not obvious from the result but the GPU particle example doesn't use alpha for the render fbo so you can't layer on top of other stuff. Simply change this line to GL_RGBA32F should be enough

https://github.com/openframeworks/openFrameworks/blob/master/examples/gl/gpuParticleSystemExample/src/ofApp.cpp#L74

(I personally prefer to use alpha channel on ping pong fbos as well so you can pass other attributes such as particle age - but maybe this is a stretch for this example)