Fluid simulation - Uniformise splat radius across screen aspects

[Keenuts]

context: https://github.com/google/bigwheels/pull/20/files/66100b1682d1ce7b4566417f90bcb1f7494c5eb3#r1060646881

When changing the aspect ratio, the simulation aspect changes because the radius of slats is only taken into account for wide screens, not tall ones.

This should be fixable by multiplying radius by aspect in all cases.