Experiment with opacity

Does not work with Wayland or XWayland so far, unfortunately.

zig/sdl/hello_sdl.zig

@@ -55,12 +55,20 @@ pub fn main() !void {
    var window_width = glyph_width * 100;
    var window_height = glyph_height * 20;
    const screen = c.SDL_CreateWindow("hello fonts", c.SDL_WINDOWPOS_CENTERED, c.SDL_WINDOWPOS_CENTERED, window_width, window_height, c.SDL_WINDOW_BORDERLESS) orelse {
    const screen = c.SDL_CreateWindow("hello fonts", c.SDL_WINDOWPOS_CENTERED, c.SDL_WINDOWPOS_CENTERED, window_width, window_height, c.SDL_WINDOW_BORDERLESS | c.SDL_WINDOW_OPENGL) orelse {
        c.SDL_Log("Unable to create window: %s", c.SDL_GetError());
        return error.SDLInitializationFailed;
    };
    defer c.SDL_DestroyWindow(screen);
    const op: f32 = 0.5;
    if (c.SDL_SetWindowOpacity(screen, op) != 0) {
        c.SDL_Log("Unable to make window transparent: %s", c.SDL_GetError());
    }
    var opacity: f32 = 10.0;
    _ = c.SDL_GetWindowOpacity(screen, &opacity);
    c.SDL_Log("opacity: %f", opacity);
    const renderer = c.SDL_CreateRenderer(screen, -1, c.SDL_RENDERER_ACCELERATED);
    var msg = "                                                                                                    ".*;
@@ -200,11 +208,13 @@ pub fn main() !void {
            }
        }
        _ = c.SDL_SetRenderDrawColor(renderer, 0, 0, 0, 100);
        //_ = c.SDL_SetRenderDrawBlendMode(renderer, c.SDL_BlendMode.SDL_BLENDMODE_BLEND);
        _ = c.SDL_RenderClear(renderer);
        // thanks to https://stackoverflow.com/questions/22886500/how-to-render-text-in-sdl2 for some actually useful code here
        const white: c.SDL_Color = c.SDL_Color{ .r = 255, .g = 255, .b = 255, .a = 255 };
        const black: c.SDL_Color = c.SDL_Color{ .r = 0, .g = 0, .b = 0, .a = 255 };
        const black: c.SDL_Color = c.SDL_Color{ .r = 0, .g = 0, .b = 0, .a = 100 };
        // Shaded vs Solid gives a nicer output, with solid the output
        // was squiggly and not aligned with a baseline.
        const text = c.TTF_RenderUTF8_Shaded(font, &msg, white, black);