Allows to share buffer data between the render result and image buffers.
The storage of the passes and buffers in the render result have been
wrapped into utility structures, with functions to operate on them.
Currently only image buffers which are sharing buffers with the render
results are using the implicit sharing. This allows proper decoupling of
the image buffers from the lifetime of the underlying render result.
Fixes#107248: Compositor ACCESS VIOLATION when updating datablocks from handlers
Additionally, this lowers the memory usage of multi-layer EXR sequences
by avoiding having two copies of render passes in memory.
It is possible to use implicit sharing in more places, but needs
some API to ensure the render result is the only owner of data before
writing to its pixels.
Sergey Sharybin