In some/many cases, an `ImBuf` is allocated, and all the pixels are
immediately filled by some code. Doing the memory clear within allocation
is just memory traffic for no good reason.
Add a flag to skip initialization of ImBuf pixels (IB_uninitialized_pixels)
and use that in some parts of VSE effects/rendering/cache/scopes, as well
as image loading code.
Rendering out VSE movie, on Windows/VS2022/Ryzen5950X:
- Sprite Fright: 443sec -> 414sec (takes 93% of previous time)
- Gold previs: 367sec -> 325sec (takes 88% of prev time)
Pull Request: https://projects.blender.org/blender/blender/pulls/118321
b4c6c69632