Re: KMS/DRM context for the GL/GLES renderer

I've recently been working on a KMS/DRM videodriver for SDL2, in this branch on Github: https://github.com/sigmaris/SDL-mirror/tree/kmsdrm
I've been copying the Raspberry Pi videodriver quite closely.
It doesn't use anything from DirectFB - only libdrm, GBM, and Mesa EGL.
My motivation is basically to get EmulationStation from RetroPie working, using this videodriver, without X11 on a Raspberry Pi 3 (not using dispmanx but using Mesa and Eric Anholt's open-source vc4 drivers).
It's at an early-ish stage, but it's working to the extent that EmulationStation can be built against SDL2 with this driver, and starts up and shows its UI on the frame buffer.

A few things are missing or still to be done, though, and there are bugs.

* There's no support for mouse cursor yet. It could be done with drmModeSetCursor on platforms that support hardware cursors, but I don't know what to do for a software fallback.
* There's no support for changing the screen mode with SDL.
* There's no support for multiple displays.
* It'd be nice to have dynamic loading support for libdrm/libgbm/etc, like the Wayland and X11 videodrivers.
* There's no way to have more than one (fullscreen) SDL window (Maybe this is OK as a limitation of this platform though)
* I've only added it into the CMake build system, not the autotools system.


Right now I'm investigating a problem where EmulationStation shuts down its SDL video system (calls SDL_VideoQuit) and launches another program, then when that program finishes, ES re-initialises its SDL video. After it re-inits the video subsystem, I keep getting Access Denied when calling drmModePageFlip(). So it works the first time, but not after quit and re-init, even though I close the /dev/dri/card0 file descriptor in VideoQuit and re-open it in VideoInit.

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