Improve window handling in the SDL backend

[funchal]

SDL_Window* is stored in a global (g_Window) by ImGui_ImplSDL2_Init. However some apis in the SDL backend use the global and some don't, creating potential for inconsistency/mistakes when starting a new frame, or when processing events. This change makes the SDL backend always use the window passed to ImGui_ImplSDL2_Init originally.

[ocornut]

Hello,

I get the removal of the window parameter to NewFrame() function (I'd accept it as a breaking change), but I think the windowID filtering, albeit correct here, would conflict with how the multi-viewport feature in docking branch uses this function. It's just easier to leave it unfiltered.

Did you need the filter or did you add the filter for correctness?

[funchal]

In my own code I do filter the events, but I wasn't aware of conflict with the docking branch, so I just reverted the windowID filtering change. I think getting rid of the window parameter is good enough on its own.

[funchal]

Fixed the merge conflicts, @ocornut do you think this is worth merging?

[ocornut]

Yes probably will merge eventually, sorry been juggling with lots of PR :)

[ocornut]

Hello @funchal ,
I have now merged 6792e1a with include your change + a temporary inline redirection function.
Thank you!