bitmask: Optimize no-op resizes
bitmask: Optimize no-op resizes

Statistics for the gtk3-demo listbox example show that the
vast majority of calls to _gtk_allocated_bitmask_resize go
from a size of 2 to 2. Don't needlessly call realloc() in
this case.

davidgumberg/gtk,grubersjoe/adwaita,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,grubersjoe/adwaita,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,jigpu/gtk