stop 'viewport override' logspam

[hizzlekizzle]

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

Warmenhoven's excellent overlay viewport resizing feature (5259c14) works a treat, but it also spams Applying viewport override! to logs/stdout on every frame while active. This PR just sets a flag to suppress any further copies of the message until the next overlay init.

Related Issues

none that I know of

Related Pull Requests

none

Reviewers

@warmenhoven