operations: Fix plasma.c's implementation

- Make it a GeglOperationSource
- Attributing works legitimately
- Don't leak GRand
- Works on R'G'B' instead of RGBA
- Try to stay in 80 columns
- Increase tile size to 512x512 to improve performances
- Some minor format fixes
operations: plasma.c: workshop -> common
Follow plasma.c's location in POTFILE.in