Patch: two fixes to the circle graphics' centre dot, no API change.
Fixed:
- The centre dot showed in every mode, not just the edit modes. A regression in
1.5.0: publishing the base feature for the point-anchored holders left them as
plain features with no style function, so OpenLayers fell back to its own
default style, which paints a dot and cannot consult `hidden`. 29 features
were affected. They are now built by createCenterBaseFeature(), which supplies
the hidden-aware style and clears `base` so Modify still ignores them.
- The centre dot now moves the graphic. It stays refused for resize (the scale
ratio divides by distance-to-centre, ~0 there) and rotate (a point on the axis
carries no angle), but translate applies a plain delta and has neither problem.
Its colour follows grabbability so grey keeps meaning "not draggable now".
300 tests.