Fix (unreported) bad handling of brush's fill threshold value.

'1' threshold value would only allow to access a third of the basic
'color space' (from black to white, from 0.0 to 1.0 component values),
when you expect it to access the whole range.

Unfortunately, this needs a subversion bump to allow already defined
brushes to keep exact same behavior!

Also, did not change default value (0.2) for new brushes, think here
keeping current one makes more sense.

Thanks to @LucaRood for confirming the issue.

source/blender/blenkernel/BKE_blender_version.h

@@ -28,7 +28,7 @@
  * and keep comment above the defines.
  * Use STRINGIFY() rather than defining with quotes */
 #define BLENDER_VERSION         278
-#define BLENDER_SUBVERSION      3
+#define BLENDER_SUBVERSION      4
 /* Several breakages with 270, e.g. constraint deg vs rad */
 #define BLENDER_MINVERSION      270
 #define BLENDER_MINSUBVERSION   6

source/blender/blenloader/intern/versioning_270.c

@@ -1466,4 +1466,11 @@ void blo_do_versions_270(FileData *fd, Library *UNUSED(lib), Main *main)
 			}
 		}
 	}
+
+	if (!MAIN_VERSION_ATLEAST(main, 278, 4)) {
+		const float sqrt_3 = (float)M_SQRT3;
+		for (Brush *br = main->brush.first; br; br = br->id.next) {
+			br->fill_threshold /= sqrt_3;
+		}
+	}
 }

source/blender/editors/sculpt_paint/paint_image_2d.c

@@ -1489,7 +1489,8 @@ void paint_2d_bucket_fill(
 		float image_init[2];
 		int minx = ibuf->x, miny = ibuf->y, maxx = 0, maxy = 0;
 		float pixel_color[4];
-		float threshold_sq = br->fill_threshold * br->fill_threshold;
+		/* We are comparing to sum of three squared values (assumed in range [0,1]), so need to multiply... */
+		float threshold_sq = br->fill_threshold * br->fill_threshold * 3;
 
 		UI_view2d_region_to_view(s->v2d, mouse_init[0], mouse_init[1], &image_init[0], &image_init[1]);