Some persistent data code was disable due to a deeper design issue, which
meant some updates were not communicated to renderers.
Dependency graph updates work in two passes, once where Blender scene
animation updates are done, then app handler scripts can run to make further
scene modifications, and then the depsgraph is updated again to take those
into account.
Previously the viewport would update renderers twice when such app handler
scripts were present. Now both viewport and persistent data rendering update
the renderers only once, accumulating updates from both passes.
For Cycles, when enabling the Persistent Data option, the full render data
will be preserved from frame-to-frame in animation renders and between
re-renders of the scene. This means that any modifier evaluation, BVH
building, OpenGL vertex buffer uploads, etc, can be done only once for
unchanged objects. This comes at an increased memory cost.
Previously there option was named Persistent Images and had a more limited
impact on render time and memory.
When using multiple view layers, only data from a single view layer is
preserved to keep memory usage somewhat under control. However objects
shared between view layers are preserved, and so this can speedup such
renders as well, even single frame renders.
For Eevee and Workbench this option is not available, however these engines
will now always reuse the depsgraph for animation and multiple view layers.
This can significantly speed up rendering.
These engines do not support sharing the depsgraph between re-renders, due
to technical issues regarding OpenGL contexts. Support for this could be added
if those are solved, see the code comments for details.
Baking vertex colors per-corner leads to unwanted discontinuities when there is
sampling noise, for example in ambient occlusion or with a bevel shader node for
normals. For this reason the code used to always average results per-vertex.
However when using split normals, multiple materials or UV islands, we do want to
preserve discontinuities. So now bake per corner, but make sure the sampling seed
is shared for vertices.
Fix T85550: vertex color baking crash with split normals, Ref D10399
Fix T84663: vertex color baking blending at UV seams
Split of internal/external image bake target code off into smaller functions and
refactor associated data structures for clarity. Designed so that a vertex color
bake target is easy to fit in.
Also avoid passing in a huge number of arguments into the main baking function,
pass a struct instead.
