Steps to reproduce:

Project:
1. Open attached project
2. Open the 'DemoScene2D' scene in 'Assets/WeatherMaker/Demo/'
3. Build and Run with WebGL Player
4. Toggle 'Rain' or 'Snow' and increase the 'Intensity' slider to maximum
5. Notice that the particles collide with the mesh appropriately
6. Toggle 'Collision' off and on again
7. Notice that the particles are no longer being generated when 'Collision' is on

WeatherMaker Demo:
1. Open http://demo.digitalruby.com/weathermaker/demo3d/
2. Toggle 'Rain' or 'Snow' and increase the 'Intensity' slider to maximum
3. Notice that the particles do not collide with the ground and the WebGL player starts to stutter

Expected: When 'Collision' is toggled on, particles stop when they collide with the ground and fade away

Not reproducible: 5.3.4p4, 5.4.0b11, (cannot test other versions due to WebGL incompatibility with browser)
Regression: from 5.4.0b14 to 5.4.0b15
Reproducible: 5.4.0b15, 5.4.0b16