examples: make sure some structs has guaranteed in-memory layout

examples/pixel-post-process/cube_mesh.zig

@@ -1,4 +1,4 @@
-pub const Vertex = struct {
+pub const Vertex = extern struct {
     pos: @Vector(3, f32),
     normal: @Vector(3, f32),
     uv: @Vector(2, f32),

examples/pixel-post-process/main.zig

@@ -16,7 +16,7 @@ const pixel_size = 8;
 const UniformBufferObject = struct {
     mat: zm.Mat,
 };
-const PostUniformBufferObject = struct {
+const PostUniformBufferObject = extern struct {
     width: u32,
     height: u32,
     pixel_size: u32 = pixel_size,

examples/pixel-post-process/quad_mesh.zig

@@ -1,4 +1,4 @@
-pub const Quad = struct {
+pub const Quad = extern struct {
     pos: @Vector(3, f32),
     uv: @Vector(2, f32),
 };