Make sure existing OpenGL state is respected.

[reworks-org]

Only done for GL3 backend for now.

Partially resolve #437

Still need to add checks around where EnableScissorRegion is called by RmlUi.

[mikke89]

Thanks! This is a welcome addition.

I want to clarify one aspect here: I agree with setting and backing up the state during the begin- and end frame calls. However, for performance reason I think it's a good idea to reserve the ability to change the GL state between render calls. The changes made in this PR is perfectly in line with this though, so it doesn't involve any further changes here. However, this means we at some later point might, for example, leave the current program in a non-zero state, and expect it to remain that way for the next render call.

I'm not sure I understand what you mean with the internal changes needed regarding the scissoring. Following the above clarification, we only need to initialize, and restore the state during the BeginFrame/EndFrame calls.

Could you also add the backup/restore for the blending and stencil functions? I'd rather merge this without any TODOs.

[reworks-org]

Doh, just realized it doesn't matter what changes are done by rml during rendering, since we have already backed it up and are restoring after anyway...

[reworks-org]

pushed changes. Should fully resolve #437

[mikke89]

Hey, I just added some more GL state to the backup, and did some refactoring. Let me know if you find any issues, otherwise I will merge it like this.

[reworks-org]

lgtm

[mikke89]

Great, thank you for the PR.