rewrite of the texture and framebuffer handling

CMakeLists.txt

include/scigl_render/buffer/frame_buffer.hpp

include/scigl_render/buffer/frame_buffer_reader.hpp

include/scigl_render/buffer/texture2d.hpp

include/scigl_render/example/depth_offscreen_render.hpp

include/scigl_render/render/texture_fullscreen_render.hpp

src/buffer/frame_buffer.cpp

src/buffer/frame_buffer_reader.cpp

src/buffer/texture2d.cpp

src/example/depth_offscreen_render.cpp

src/example/scigl_depth_viewer.cpp

src/render/texture_fullscreen_render.cpp