- clamp scissors fully to avoid NVidia's awful drivers locking up the entire system if they end up out of bounds
- perform buffer clears as part of the render pass. this puts some restrictions on how FRenderState.Clear can be used
- add an offset uniform to the present shaders so the vulkan target can flip the image during presentation
* 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.
Unlike uObjectColor2, this is more global state. uObjectColor2 is part of gradient calculation which may later be offloaded to a secondary buffer which already resulted in only conditionally setting it, resulting in broken special colormap application for the software renderer.
