UI: add non-linear slider support

This patch introduces non linear sliders. That means, that the movement
of the mouse doesn't map linearly to the value of the slider.

The following changes have been made.

- Free logarithmic sliders with maximum range of (`0 <= x < inf`)
- Logarithmic sliders with correct value indication bar.
- Free cubic sliders with maximum range of (`-inf < x < inf`)
- Cubic sliders with correct value indication bar.

Cubic mapping has been added as well, because it's used for brush sizes
in other applications (Krita for e.g.).

To make a slider have a different scale type use following line in RNA:
 `RNA_def_property_ui_scale_type(prop, PROP_SCALE_LOGARITHMIC);`
or:
 `RNA_def_property_ui_scale_type(prop, PROP_SCALE_CUBIC);`

Test the precision, step size and soft-min if you change the scale type
of a property as it will feel very different and may need tweaking.

Ref D9074

1 files changed, 8 insertions, 0 deletions

diff --git a/source/blender/editors/interface/interface.c b/source/blender/editors/interface/interface.c
index 90d604b3190..a31efefd99c 100644
--- a/source/blender/editors/interface/interface.c
+++ b/source/blender/editors/interface/interface.c
@@ -2327,6 +2327,14 @@ bool ui_but_is_float(const uiBut *but)
   return false;
 }
 
+PropertyScaleType ui_but_scale_type(const uiBut *but)
+{
+  if (but->rnaprop) {
+    return RNA_property_ui_scale(but->rnaprop);
+  }
+  return PROP_SCALE_LINEAR;
+}
+
 bool ui_but_is_bool(const uiBut *but)
 {
   if (ELEM(but->type,