Fix#121782
`GPencil` engine renders on the whole framebuffer.
If `Render region` is used, the result of GP rendering is scaled from whole
framebuffer down to the selected region.
The fix adds framebuffer viewport reset after rendering.
This resolves the problem for downstream consumers of
the framebuffer (e.g. `GPencil`).
The final result is somewhat similar to the `EEVEE Legacy`.
Now `EEVEE` rendering is done in the selected region, while `GPencil`
is rendered on whole viewport as before.
Pull Request: https://projects.blender.org/blender/blender/pulls/121979
7d5cd60023