Allow the user to choose the resolution when exporting a mask

Currently, when exporting out an individual mask, the export dialog box only allows the user to pick a file format, and uses the current document size for the resolution. It could be useful to allow the user to pick a different export resolution- that way for performance reasons they could be working in their file at a lower resolution than the final intended texture size, and be able to export out an individual mask at that final, higher resolution without having to change the size for the whole project and wait for everything to update.