Now that we support basic LWRP, switching between HDRP and LWRP ought to update the render pipeline settings path.

Repro steps:
1. Open an HDRP-enabled VFX project
2. Pull LWRP
3. Switch the current SRP to LWRP
4. Observe the particles don't work
5. Go to Project Settings/VFX and change the end of the path from "RenderPipeline/HDRP" to "RenderPipeline/LWRP"
6. Recompile particles and observe that they now work

Notes:
- should automatically switch between officially-supported SRPs
- currently if the RP settings path is wrong, a very unhelpful error pops up:
Exception while compiling expression graph: System.ArgumentException: Illegal characters in path. This should be replaced with a message instructing to check the render pipeline settings path in Project Settings/VFX.
- The field itself should probably be renamed to Render Pipeline Settings Path (currently it's "Render Pipe")