this is an experimental complete rewrite of darktable, naturally at this point
with a heavily reduced feature set.

the processing pipeline has been rewritten as a generic node graph (dag) which
supports multiple inputs and multiple outputs. all processing is done in glsl
shaders/vulkan. this facilitates potentially heavy duty computational
photography tasks, for instance aligning multiple raw files and merging them
before further processing, as well as outputting many intermediate results as
jpg for debugging. the gui profits from this scheme as well and can display
textures while they are still on gpu and output the data to multiple targets,
such as the main view and histograms.