Early return optimisation for volume rendering with tricubic interpolation + adjustable sampling rate multiplier

[mlavik1]

Early return optimisation for volume rendering with tricubic interpolation + adjustable sampling rate multiplier

Two changes:

Optimised volume rendering tricubic interpolation

We now fetch the density value without tricubic interpolation first, to see if we can do an early return.

This has a minimal cost, but for transfer functions where the lowest density values are mapped to zero then the performance benefit is great (up to 2-3 times faster).

Sampling rate multiplier setting

Previously I've advised users on very low-end devices (or AR) to modify the "MAX_NUM_STEPS" values in the shader directly, to improve rendering performance at the cost of visual quality. Now there's a setting for this: