Black screen on non-retroarch frontend

[rtomasa]

Hi I'm developing a new libretro frontend based on OpenGL ES 3.1
It seems that the core is always using a framebuffer with Id=4 no matter what you pass via core_get_current_framebuffer from RETRO_ENVIRONMENT_SET_HW_RENDER.
Also the image of this buffer is totally corrupted if you force to display the same. Other glcores like Flycast and Muppen 64 plus run well in my front end, so not sure if there is something else special to do with this one.