[Bf-funboard] Crossfade from Eevee when updating the viewport in Cycles

[Tiago Estill de Noronha]

What if during the period the Cycles realtime resolution is very low, and
maybe for the first N-percent of the samples, the viewport was
simultaneously rendered in Eevee, and the picture gradually crossfaded
between the renderers, to more immediately show updates? It doesn't matter
that Eevee isn't 100% accurate, as during the low resolution and low
samples period you're not exactly getting a very accurate render from
Cycles either way.

This of course would need to be optional; though it probably would be good
to be enabled by default since discrepancies between Eevee and Cycle
materials tend to mostly only be present on more advanced cases.