Brush: default the strength to 1.0

This applies to all paint modes except sculpt and grease pencil brushes.

When painting color or weight it's best to paint the color
the user has selected, without them having to make multiple strokes.

source/blender/blenloader/intern/versioning_250.c

@@ -1696,7 +1696,7 @@ void blo_do_versions_250(FileData *fd, Library *lib, Main *bmain)
 
       /* will have no effect */
       if (brush->alpha == 0) {
-        brush->alpha = 0.5f;
+        brush->alpha = 1.0f;
       }
 
       /* bad radius */