It is not possible to flip images using nk_image and nk_subimage

[bardowastaken]

Hello,
nk_subimage_id is casting the specified region components to nk_ushort which makes it impossible to use negative sizes on images.
Using opengl, framebuffer textures have their origin on the bottom left, which requires me flip the y axis when rendering them in the ui, which is not possible currently.
This seemed like an oversight to me, but if it is done on purpose, ignore me.