SDL3: fix renderer logical presentation

[cheyao]

This pull request fixes some clipping issues and mouse offset issues (should it be called this?) with SDL's logical presentation when using sdlrenderer back-end with SDL3.

Here is a video of the behavior after the change and before the change: (Sorry for the bad video quality)
https://imgur.com/a/w9XrzaJ

You can test this change by going to examples/example_sdl3_sdlrenderer3 and adding this after initializing the renderer:

SDL_SetRenderLogicalPresentation(renderer, 1024, 768,
					SDL_LOGICAL_PRESENTATION_LETTERBOX,
					SDL_SCALEMODE_BEST);

and this right after polling the event

SDL_ConvertEventToRenderCoordinates(renderer, &event);

[ocornut]

Hello,
Sorry for not reacting to this yet. We have dozens of similar changes desirable in various backends, partly related to DPI handling but also related to the general possibility of separating imgui coordinates from window coordinates.
SDL3 also supporting this feature add one extra layer into the mix. Please give me some time until I can investigate this (probably after 1.91).