chore: handle all cases of enum

src/main.zig

@@ -11,6 +11,5 @@ pub const plot = @import("plot.zig");
 pub const backend = switch (@import("zgui_options").backend) {
     .glfw_wgpu => @import("backend_glfw_wgpu.zig"),
     .glfw_opengl3 => @import("backend_glfw_opengl3.zig"),
-    .win32_dx12 => .{}, // TODO:
-    .no_backend => .{},
+    .no_backend, .win32_dx12, .sdl2_opengl3 => @panic("unsupported backend"),
 };