Increase fluid buffer size

[griffi-gh]

closes #67

increases the max amount of fluid by 16 times.

• fixed mesh->i32 (32 bit buffer) support
• switched to a i32 mesh for fluids
• increased box2d particle limit to 65536
• increased fluid buffer size to match the box2d limit

doesn't break compatability completely (levels can still be loaded) but can affect amount of water spawned.

waiting for the version code increase

letest comit checks for <= 1.5.1

[rollerozxa]

merged as 5bc55aa0f5dc85e5df6f7d8f2a0b89bb1430a189