Add underlay functionality to OGL1/2/3 and vulkan drivers

[hunterk]

Guidelines

1. Rebase before opening a pull request
2. If you are sending several unrelated fixes or features, use a branch and a separate pull request for each
3. If possible try squashing everything in a single commit. This is particularly beneficial in the case of feature merges since it allows easy bisecting when a problem arises
4. RetroArch codebase follows C89 coding rules for portability across many old platforms check using C89_BUILD=1

Description

A lot of people use overlays cosmetically for borders/bezels, and they currently have to include a transparent cutout to match the content exactly, which means separate overlays for integer and non-integer, and no freedom to change aspect ratios from, say, nominal 4:3 to 8:7 (1:1 PAR for S/NES) without introducing either black borders or cutting off game content.

This PR adds a "overlayX_background_fill" flag to the overlay subsystem to allow users to put an overlay behind the game image instead of on top of it, so the overlay gets obscured when the game image changes size instead of the other way around.

I haven't yet added this to any of the D3D drivers because I don't have any good way of testing them, and they're currently experiencing a lot of churn.

Related Issues

closes #9536 and closes #15138

Related Pull Requests

none

Reviewers

@sonninnos and @LibretroAdmin (in case you'd like to add support to the D3D drivers)