Native playground platform with a from-scratch software rasterizer

Like elm_playground_native, but every pixel is computed by our own code instead of Cairo; SDL only opens the window, reads input, and shows the pixels. Meant for teaching: the algorithms (framebuffer, affine transforms, polygon filling, Bresenham lines, midpoint circles, image warping, compositing, antialiasing, a stroke font) are written to be read, with references to the papers that introduced them. It is also the 2D platform of the native 3D programs, whose HUD it draws, and installs the SDL event loops shared by all the native platforms.