[Linux] Set the window icon in a more compatible way

[nmlgc]

Explained in the blog post. Support for manually transferred window icons is way more widespread in X11 window managers than our current indirect method via the window class, but SDL_SetWindowIcon()'s single image parameter is inadequate for pixel art that exists at multiple sizes. This would add the raw X11 code for manually transferring our window icon at all of its size variants, which allows the WM to (hopefully) display the closest variant.

For Wayland, the window class method should be enough, but we might still want to issue a xdg_toplevel_icon_v1::set_name() request for good measure.

Alternatively, we could make this more compatible kind of icon support exclusive to the SDL 3 platform layer (#68), and help out the greater community by adding alternate image support to SDL_SetWindowIcon().