Screen scaling proposal (RFC)

[anyputer]

Proposal

It would be great if we had two new environment commands: RETRO_ENVIRONMENT_GET_RENDERED_HEIGHT and RETRO_ENVIRONMENT_GET_RENDERED_ASPECT_RATIO.
These would allow hardware rendered cores like N64, PlayStation, Wii emulators to output at the window's resolution!
Owners of ultrawide, ultra HD monitors get to use the aspect ratio and/or resolutions of their amazing screens.
Standalone emulators have these features and they're missing in RetroArch.

RETRO_ENVIRONMENT_GET_RENDERED_HEIGHT - Gets the window height that can be overridden by the frontend
RETRO_ENVIRONMENT_GET_RENDERED_ASPECT_RATIO - Gets the aspect ratio of the window that can be overridden by the frontend

Examples

ParaLLEl N64 can have a resolution option: "Auto" to render at the frontend-provided resolution. It would calculate the width based on the height and aspect ratio of the game.
Dolphin has options to render at a custom aspect ratio and pixel height in the standalone emulator. In this case the core can have "Auto" core options for resolution and aspect ratio.

Proposed Definitions

#define RETRO_ENVIRONMENT_GET_RENDERED_HEIGHT (61 | RETRO_ENVIRONMENT_EXPERIMENTAL)
                                            /* unsigned * --
                                            * Integer value that lets us know the height of the window.
                                            *
                                            * The returned value can be used by cores to automatically
                                            * scale the resolution how it sees fit.
                                            *
                                            * The returned value can be NULL. In this case the core should
                                            * use a sane default (i.e. native resolution in an emulator).
                                            *
                                            * A frontend could override this if desired.
                                            */
#define RETRO_ENVIRONMENT_GET_RENDERED_ASPECT_RATIO (62 | RETRO_ENVIRONMENT_EXPERIMENTAL)
                                            /* float * --
                                            * Float value that lets us know the aspect ratio of the window.
                                            *
                                            * The returned value can be used by cores as a suggestion
                                            * for scaling.
                                            *
                                            * The returned value can be NULL.
                                            *
                                            * A frontend could override this if desired.
                                            */

Unresolved Questions

• Is rendered a good word for it? Maybe screen, window... the values can be overridden, and the doc comments should reflect that.
• Would it be better to tackle Integer core configuration field #41 first? I don't think so.
• Does this make the existing core options for resolution (and possibly aspect ratio) pointless? The frontend would be able to provide it, and emulators can set a core option for the native resolution if necessary.
• Should we concern about rotation? I don't think so if the core needs to set it, and if RetroArch's Aspect Ratio override doesn't care either.
• What settings would RetroArch have to override this? Can we use the Aspect Ratio setting, or do we need a new one? The setting for resolution could have values like Window Height (default), Window Height / 2, Core provided (would give NULL making it up to the core to figure it out, like by giving a native resolution).
• Is the formatting in my proposed definitions correct? Does the explanation for the aspect ratio command suffice?
• Do we need some sort of callback for when the window size changes? Or does it suffice to check if the current size is different from the previous... or something. Something convenient.

[anyputer]

...what happens when the window height is greater than the width
this is flawed