Fix Cycles wrong adaptive sampling render when using sample offset

Sample offset was not accounted for in the adaptive sampling code and caused
issues, like immediately applied adaptive filtering, with non-zero values.

Differential Revision: https://developer.blender.org/D13510

Cleanup: use more modern C++ in Cycles merge operator

Use structured binding and for-each loop, remove reduntant type casts, use
find_if instead of loop.

Differential Revision: https://developer.blender.org/D13456

Cycles: support merging images rendered with adaptive sampling

This patch adds support for merging images rendered with adaptive sampling to
the merge operator (which is currently only exposed in the Python API).

To do this an sample count buffer is created for each render layer from the
sample count pass if it exists, or from the metadata otherwise. This is then
used for averaging passes.

Differential Revision: https://developer.blender.org/D13457

UI: Improve curve grid drawing code

This patch fixes assert on grid drawing. `for` loops are used instead
of `while` loops to make sure the number of lines is exact. The old
code draws lots of unnecessary lines offscreen. This bug is fixed as
well. See the patch for a comparison without a scissor test.

Differential Revision: https://developer.blender.org/D8745

UI: Fix curve widget fill artifacts

Disable antialiasing which caused artifacts.

Differential Revision: https://developer.blender.org/D8497