Fix part of #34233: bad alpha blending for 2D image painting. This is a very
old issue, the formulas here were never quite right, should all work ok now
with byte and float images.

Some differences:

* Colors with zero alpha from the background will never have an influence, so
  you don't get alpha fringes when painting over such areas. This does give
  hard edges when looking at the RGB channels alone, but there's no way to
  avoid that and fringes at the same time, same behavior as other painting apps.

* Add/Subtract/Multiply/Lighten/Darken now leave the alpha channel unchanged
  and work only the RGB channels, again same behavior as many other apps.

* Erase/Add alpha now compensates for premultiplied float images to keep the
  straight RGB colors the same.

Next: fix projection painting.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56337 954f8c5b-7b00-dc11-b283-0030488c597c
Fix #35113: solidify modifier not preserving clamp option on duplicating the
object or modifier.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56338 954f8c5b-7b00-dc11-b283-0030488c597c
"Bug" & usability fix, own collection.

Color Pickers in Blender support precision editing with holding Shift key.
Unfortunately, on first click the picker cursor moves to the mouse position,
making the precision dragging quite useless.

Now, if you hold Shift, the picker editing will only apply the delta motion
of your mouse drags, making it much nicer for fine tuning of color grades
in Sequencer or Compositor.




git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56339 954f8c5b-7b00-dc11-b283-0030488c597c