Fix vertically flipped output on the OpenGL backend

[11EJDE11]

Summary

Fixes #67
The picture was vertically flipped in fullscreen OpenGL when using the PixelArt scale filter with non-integer scaling. The final render now flips it back correctly.

Behavior and compatibility

Bug fix.

Validation

Reproduced with SUN.ini - flipped before, upright after:

[Video]
ScreenWidth=1024
ScreenHeight=768
StretchMovies=no
Fullscreen=yes
WindowWidth=-1
WindowHeight=-1
ScaleMode=PixelArt
IntegerScaling=no
VSync=no
Renderer=4
CursorScale=0

Documentation

Change record: manual/changes/opengl-prescale-flip.md.

Checklist

• The change is focused; unrelated mechanical cleanup is separate
• Compatibility effects and any migration are explicit
• A player- or modder-visible engine change carries its change record
• Validation distinguishes what passed, failed, and was not run
• No prohibited assets, binaries, SDKs, credentials, or generated output are included

[github-actions]

Development builds of 90be6ad:

• opents-Release-26630b5
• opents-Debug-26630b5

The links work without a GitHub account. Artifacts expire after 90 days, and this comment follows the latest successful build.