Support for 2d masks

## What problem does this solve or what need does it fill?

Support for masking of particular areas of the screen.

## What solution would you like?

A way to specify a 2d texture as a mask and a way to turn that mask on and off.

## What alternative(s) have you considered?

Alternatively write a custom shader but it would be nice if Bevy could support this out of the box.

## Additional context

[Notan](https://github.com/Nazariglez/notan) is doing it for example:
https://nazariglez.github.io/notan-web/examples/draw_mask_texture.html

The issue does not identify Bevy files, tests, or an entry point. Start by comparing the requested 2D texture mask and on/off control with the linked Notan example, then identify the Bevy rendering area where this belongs. Done would require an agreed implementation scope and validation that masked screen regions can be enabled and disabled.

Written by the indexing model from the issue text.