* removed unneeded includes
* moved global brightmap to GPalette
* took Build tile init code out of the FTextureManager class, because this code is not compatible with how real Build games need to go about it, making the texture manager not shareable.
* removed dependency on higher level info from the Image class.
This implementation only reads GIF, BMP and PIC formats. No animated GIF support because stb_image does not handle that.
PNG, JPG and TGA are still being handled by the existing dedicated implementations.
PSD and HDR are impractical for reading texture data and thus are disabled.
PnM could be enabled, if its identification semantics were stronger. stb_image only checks the first two characters which simply would falsely identify several flats with the right colors in the first two bytes.
This is more or less a waste product of getting stb_image to work with something actually testable, so it is just provided as-is.
* 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.
