* Store per RenderPass in RenderResult.
* Caches are cleared when starting rendering, to make more memory available
to GPU rendering.
* Caches are cleared on UI changes, when no compositing node editor and no
image editor with a render result or viewer node image is visible.
* Store 3 channel RGB passes as such, and set alpha 1 in shader.
This is an intermediate step before implementing GPU backed ImBuf, to
improve performance and figure out cache eviction.
Pull Request: https://projects.blender.org/blender/blender/pulls/108818
a88114b35e