GPencil: Fix size difference between radial control and Brush size

Due an internal scale of the brush size done by grease pencil
draw tool (this scale factor cannot be removed), the size of the
radial control is not equal to the actual stroke thickness and
this makes the radial size displayed useless.

This patch adds a new property that allows to pass the
path of the tool path and this value is used to apply
the correct scale to the radial size.

Reviewed By: mendio, frogstomp, pepeland, brecht

Differential Revision: https://developer.blender.org/D16866

source/blender/editors/include/ED_gpencil.h

@@ -645,6 +645,10 @@ void ED_gpencil_stroke_close_by_distance(struct bGPDstroke *gps, float threshold
  * Calculate the brush cursor size in world space.
  */
 float ED_gpencil_cursor_radius(struct bContext *C, int x, int y);
+float ED_gpencil_radial_control_scale(struct bContext *C,
+                                      struct Brush *brush,
+                                      float initial_value,
+                                      const int mval[2]);
 
 #ifdef __cplusplus
 }