Eeschema, fix issue when rotating a hierarchical sheet: depending on the sheet size, the rotation center was not always on grid.

Therefore, in this case, pin sheets were no longer on grid.

Fixes: lp:1764270
https://bugs.launchpad.net/kicad/+bug/1764270

eeschema/sheet.cpp

@@ -491,6 +491,10 @@ void SCH_EDIT_FRAME::RotateHierarchicalSheet( SCH_SHEET* aSheet, bool aRotCCW )
     // Rotation is made around it center
     wxPoint rotPoint = aSheet->GetBoundingBox().Centre();
 
+    // Keep this rotation point on the grid, otherwise all items of this sheet
+    // will be moved off grid
+    rotPoint = GetNearestGridPosition( rotPoint );
+
     // rotate CCW, or CW. to rotate CW, rotate 3 times
     aSheet->Rotate( rotPoint );