This patch tunes the integrator state sizing for Metal (num_concurrent_states and num_concurrent_busy_states).

On all GPUs architecture, we adjust the busy:total states ratio to be 1:4 which gives better rendering performance than the previous 1:16 ratio (independent of total state count). This gives a small performance uplift (e.g. 2-3% on M1 Ultra).

Additionally for M2 architectures, we double the overall state size if there is available headroom. Inclusive of the first change, we can expect uplift of close to 10% in future, as this results in larger dispatch sizes and minimises work submission overheads. In order to make an accurate determination of available headroom, we defer the calculation of num_concurrent_states and num_concurrent_busy_states until the time of integrator state allocation (i.e. after all of the scene data has been allocated). We also refactor alloc_integrator_soa to calculate an *exact* single-state-size in a first pass, right before allocating the integrator SoA buffers in a second pass.