core: add is_headless to specify hidden window before init (#816)

Co-authored-by: Stephen Gutekanst <stephen.gutekanst@gmail.com>

libs/core/src/Core.zig

@@ -9,6 +9,7 @@ internal: platform.Core,
 
 pub const Options = struct {
     is_app: bool = false,
+    is_headless: bool = false,
     title: [*:0]const u8 = "Mach Engine",
     size: Size = .{ .width = 1920 / 2, .height = 1080 / 2 },
     power_preference: gpu.PowerPreference = .undefined,

libs/core/src/platform/native/Core.zig

@@ -87,6 +87,10 @@ pub fn init(core: *Core, allocator: std.mem.Allocator, options: Options) !void {
     // Create the test window and discover adapters using it (esp. for OpenGL)
     var hints = util.glfwWindowHintsForBackend(backend_type);
     hints.cocoa_retina_framebuffer = true;
+    if (options.is_headless) {
+        hints.visible = false; // Hiding window before creation otherwise you get the window showing up for a little bit then hiding.
+    }
+
     const window = glfw.Window.create(
         options.size.width,
         options.size.height,