This was a missing features in EEVEE for ages which
was in fact very easy to implement.
EEVEE implements the sample override like the default
`Use` value in Cycles. It always override the sample
count if not 0. Adding a new option for changing this
behavior just like Cycles can be done later while
at the same time making the option more understandable
and its value moved to the blender's DNA.
This PR moves the UI panel to the Blender side to
be shared between Cycles and EEVEE.
Pull Request: https://projects.blender.org/blender/blender/pulls/140219
e16b6ac3da