Screen-specific masks

Often I need to use the same mask for a given screen. Right now this means copying the same mask file over and over again for each screenshot, which is a lot of work.

This pull request provides screen-specific masks. For each screen that has one, the mask is applied after the global mask and before the screenshot-specific mask. To use a screen mask, the user places a PNG file with the name of the screen (e.g. my_screen.png) in the support/masks directory.