Compile with -Wfloat-conversion

This means we'll get warnings whenever a floating point value looses
precision, e.g. gets implicitly casted to an integer. It also warns when
implicitly casting double's to float's, which arguably is less of a
problem, but there are no warning for just float/double to int.

This would have caught
https://gitlab.gnome.org/GNOME/mutter/-/issues/3530.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3822>

src/core/workspace.c

@@ -1547,7 +1547,8 @@ get_pointer_window (MetaWorkspace *workspace,
 
   return meta_workspace_get_default_focus_window_at_point (workspace,
                                                            not_this_one,
-                                                           point.x, point.y);
+                                                           (int) point.x,
+                                                           (int) point.y);
 }
 
 static gboolean