Make select tool rectangle's opacity independent from layer's opacity

[PotassiumK]

Currently, if the layer's opacity is low, the selection tool's "painted" rectangular area will be low as well. If the layer's opacity is too low, the selected area can barely be seen.

[PotassiumK]

This ties into making rectangle previews from .css for generally faster performance

[zeyadhazem]

can be fixed after implementing #203