All decisions were done deep inside the texture creation code, leaving zero options to the higher level code for controlling the feature.
Changed this so that the option to upscale must be passed as a parameter to FRenderState::SetMaterial and extended all needed variables to manage the added texture variants.
Still not solved: Material layers need explicit control, not only for scaling but also for filtering.
This shouldn't be in the hardware independent interface because the semantics on OpenGL and Vulkan are too different, so a common implementation is not possible.
Most bind calls were in the GL interface anyway, so these no longer pass through hardware independent code.
This also moves the bind calls in the shadowmap code into the GL interface - these never did anything useful in Vulkan and aren't needed there.
Last but not least, this moves the legacy buffer binding handling into FGLRenderState and performs the initial binding for the light buffer in a more suitable place so that this doesn't have to pollute the render state.
* re-added screen blends for images from the hardware renderer.
* moved all postprocessing of the image out of the renderers.
* cleaned out a large piece of cruft for handling the palette in the frame buffer class. This was all a remnant of the old paletted backend that no longer exists. Nowadays the screen blend is just a postprocessing effect drawn over the 3D screen, there is no need to maintain any of it as global state anymore.
* since the engine doesn't produce paletted screenshots anymore there is no need to have handling for it in the generation code. This depended on otherwise obsolete information so it got removed along with that information.
