Adds some support for "less giant" level screenshots.
* In the Editor, the Level->Take Screenshot menu will render a cropped screen
shot of just the level viewport on screen. Note: it is not an SDL2 screen
copy but generated from scratch from the level data.
* In levels themselves, screenshots can be stored inside the level data in
three different sizes: large (1280x720), medium and small (each a halved
size of the previous).
* The first screenshot is created when the level is saved, starting from
wherever the scroll position in the editor is at, and recording the 720p
view of the level from there.
* The level screenshot can be previewed and updated in the Level Properties
window of the editor: so you can scroll the editor to just the right position
and take a good screenshot to represent your level.
* In the future: these embedded level screenshots will be displayed on the
Story Mode and other screens to see a preview of each level.
Other tweaks:
* When taking a Giant Screenshot: a confirm modal will warn the player that
it may take a while. And during the screenshot, show the new Wait Modal to
block player interaction until the screenshot has finished.
Noah Petherbridge
da83231559