* Fix several wrong coordinates, causing things to be drawn in the wrong places.
* Remove unexpected return.
* Slight peformance improvement, by reducing number of shader binds.
* Minor code style stuff.
Part of T49043
Basically all this does is drawing layout previews into the opened layout search menu.
https://youtu.be/RHYWtZP7pyA
The previews are drawn using offscreen rendering so they can't use multi-threading (yet!). But that shouldn't be an issue since only a handful of previews are drawn at the same time. Normally we only need to redraw the preview if a screen layout was changed. Would be nice if PreviewImage could store if it supports threaded rendering.
Previews are saved in files, might be useful if you later want to support appending layouts.
Adds a new file screen_draw.c.
