ShumateMapLayer currently breaks the GTK4 allocation model by calling
gtk_widget_size_allocate() outsize allocation, and by queueing allocation
during allocation (by removing and adding widgets).

Rework its allocation to not do that anymore. Make the allocation vfunc
only allocate, and never queue allocations.

Despite the scary diff, this change merely moves the contents of the
shumate_map_layer_compute_grid() function to inside the allocation
vfunc, and the contents of the allocation vfunc to recompute_grid().