fix: add bounding sphere to grass shader demo

[isaac-mason]

• Add bounding sphere to grass shader demo
• Addresses Grass shader bug #81

[abernier]

hey @isaac-mason thx, is it ready for merge?

[hichemfantar]

Thanks for the pr! I'll test and report back asap

[hichemfantar]

The bug seems to be fixed!
I have a question, is there any performance impact with adding a bounding sphere?
Also the entire grass shader example seems to have performance pitfalls according to this thread.
Do you agree that we should update the implementation?