Theme: Radial gradient background and enum for gradient type

This commit replaces the "Use Gradient" checkbox theme option with an
enum and implements a radial background.

Whith this change, it should be easier to implemet other types of more
complex background types, like a world space oriented gradient.

Reviewed By: billreynish, fclem, brecht

Differential Revision: https://developer.blender.org/D6825

Fix T54270: Reset last_hit and last_location when reading the file

It does not make sense to read those values when loading a file and they can crash the cursor if they contain invalid coordinates.

Reviewed By: brecht

Maniphest Tasks: T54270

Differential Revision: https://developer.blender.org/D6754

Sculpt: Pinch only in the direction perpendicular to the stroke

By pinching this way, we can fix some artifacts when sculpting following
the topology direction. It does not make much difference with dyntopo/
remesher, but I think this should improve the quality of the brush when
working with Multires.

Reviewed By: JulienKaspar, jbakker

Differential Revision: https://developer.blender.org/D6587

Sculpt: Update red cursor color

Some users reported that the previous red color of the cursor was too
saturated and that can be distracting.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6828

Fix T73105: Use Magnify instead of Pinch in the brush context menu

This matches the name in the properties panel for the Blob and Snake
Hook brushes

Reviewed By: brecht

Maniphest Tasks: T73105

Differential Revision: https://developer.blender.org/D6805

Sculpt: Fix slide relax cursor color

Slide/Relax is a deform tool, so the color should be yellow.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6827

Sculpt: Split normal radius and area radius

This enables an extra layer of control in the sculpt brushes.
For now it is enabled only in Scrape, but it should work in all brushes (like normal radius). In the future it may also be enabled in other brushes.
You can tweak in this property in the scrape brush to achieve a much better behavior when working on curve surfaces and control how much volume you want to trim. In most cases, it also fixes the bug where the brush keeps trimming in the same area without disabling accumulate.
It should be possible to fix some other artifacts in other brushes by tweaking this default property.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D5993

Sculpt: Clay Strips Tip Roundness property

This patch allow to change the brush tip shape between a square and a
circle using a brush property.

After this change we are no longer testing the distance against a cube
(the Z axis is not used). I did not test this in depth, but if it does
not produce any artifacts I think we can keep it this way instead of
adding more complexity to the code.

In this new distance test the brush falloff is only applied on the
rounded parts of the square to avoid sharp artifacts in the
diagonals. Because of this, the round version is much softer than
the square one. The planned hardness property will fix this, but
this can also be avoided by setting the fallof to a custom curve.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D6165

Pose Brush: Option to disable the IK anchor point

The IK chain was using an anchor point by default as it makes sense for
posing, but for creating curved shapes it is useful to be able to
disable it.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D6584

Sculpt: Clay Thumb Brush

This brush simulates deforming clay with your fingers, accumulating
material during the stroke. It has a plane that tilts during the stroke
in the front part of the brush to achieve this effect.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D6238