filmulate.cpp

First filmulate sets up the film and developer itself.

Then, it runs exposure() to convert the brightness map into a count of 'active crystals per pixel'. This is akin to exposing actual film with light, which creates a number of seeds in proportion to the brightness.

Then, it runs a loop that simulates development. develop() consumes developer and silver salts, growing the crystals. diffuse() implements diffusion within the active layer of developer. layer_mix() implements diffusion between the active layer of developer and the reservoir of developer. agitate(), which occurs only once every few cycles of development, completely homogenizes the active layer of developer with the reservoir.