FLTK: Don't oops() when unknown mouse button is pressed.

For example forward/backward buttons are present on many mouses
but are not used by solvespace. This oops() caused solvespace to
crash whenever those buttons were (accidentally) pressed.

src/fltk/fltkmain.cpp

@@ -457,7 +457,7 @@ public:
                     case MOUSE(FL_RIGHT_MOUSE, FL_RELEASE):
                         SS.GW.MouseRightUp(x, y); break;
 
-                    default: oops(); break;
+                    default: break;
                 }
 #undef MOUSE
                 return 1;