Allow Camera::SetFlipVertical to be used externally for OpenGL

[ChrisDenham]

Allow Camera::SetFlipVertical to be used externally for OpenGL by toggling it either side of render to texture, i.e. instead of "set and clear" either side of render to texture.

The purpose of this pull request is to allow Camera::SetFlipVertical to be used for both DirectX and OpenGL to switch into a right handed coordinate system (see #2642).

This pull request should not change the behaviour of any application that does not call Camera::SetFlipVertical directly. It also should correct the scenario where if the render to texture should not be flipped if the camera is already flipped (hence the toggling of state in the changes)

Closes #2642

[eugeneko]

If no objections arise, I'll merge it in three days

[Lumak]

for every render target. flipping is toggled

camera_->SetFlipVertical(!camera_->GetFlipVertical());

so every other render target is flipped?

just looked at the code, and this exists in Render() function, so it's flipped every other frame?

[ChrisDenham]

No, I don't think so. My change toggles the flip camera state either side of the render for OGL. I believe this is equivalent to the original code which sets then clears the flipped state. The purpose of my PR is simply to retain the flipped state of the camera set by the user, which was previously being lost every frame for OGL.

[1vanK]

Yes, the code does not look intuitive. May be add some more comment, that the value restored below.

[ChrisDenham]

Yes, the code does not look intuitive. May be add some more comment, that the value restored below.

Yes, I agree, it's not immediately obvious why it's a toggle or why the restoration is a toggle.
How's this amended comment?

#ifdef URHO3D_OPENGL
    if (renderTarget_)
    {
        // On OpenGL, flip the projection if rendering to a texture so that the texture can be addressed in the same way
        // as a render texture produced on Direct3D9
        // Note that the state of the FlipVertical mode is toggled here rather than enabled
        // The reason for this is that we want the mode to be the opposite of what the user has currently set for the
        // camera when rendering to texture for OpenGL
        // This mode is returned to the original state by toggling it again below, after the render
        if (camera_)
            camera_->SetFlipVertical(!camera_->GetFlipVertical());
    }
#endif

[Lumak]

Yes, the comment is very informative. I get it now.

[ChrisDenham]

Can this PR be merged now? Or are there any outstanding concerns about it?

[eugeneko]

Yeah, I tend to say "I'll merge it in 3 days" and forget about it for weeks.