Added texture atlas example

examples/texture_atlas.rs

+use bevy::{
+    prelude::{App as BevyApp, AssetServer, Commands, Handle, Res, ResMut},
+    window::WindowDescriptor,
+    DefaultPlugins,
+};
+use kayak_core::styles::PositionType;
+use kayak_ui::bevy::{BevyContext, BevyKayakUIPlugin, ImageManager, UICameraBundle};
+use kayak_ui::core::{
+    render,
+    styles::{Style, StyleProp, Units},
+    Index,
+};
+use kayak_ui::widgets::{App, TextureAtlas};
+
+fn startup(
+    mut commands: Commands,
+    asset_server: Res<AssetServer>,
+    mut image_manager: ResMut<ImageManager>,
+) {
+    commands.spawn_bundle(UICameraBundle::new());
+
+    let image_handle: Handle<bevy::render::texture::Image> = asset_server.load("texture_atlas.png");
+    let ui_image_handle = image_manager.get(&image_handle);
+
+    //texture_atlas.png uses 16 pixel sprites and is 272x128 pixels
+    let tile_size = 16;
+    let columns = 272 / tile_size;
+    let rows = 128 / tile_size;
+    let atlas = bevy::sprite::TextureAtlas::from_grid(image_handle, bevy::prelude::Vec2::splat(tile_size as f32), columns, rows);
+
+    //The sign in the top right of the image would be index 16
+    let sign_index = 16;
+    //The flower is in the 6(-1) row and 15 collumn
+    let flower_index = columns * 5 + 15;
+
+    let context = BevyContext::new(|context| {
+        let atlas_styles = Style {
+            position_type: StyleProp::Value(PositionType::ParentDirected),
+            width: StyleProp::Value(Units::Pixels(200.0)),
+            height: StyleProp::Value(Units::Pixels(200.0)),
+            ..Style::default()
+        };
+
+        let rect = atlas.textures[sign_index];
+        let sign_position = rect.min;
+        let sign_size = rect.max - rect.min;
+
+        let rect = atlas.textures[flower_index];
+        let flower_position = rect.min;
+        let flower_size = rect.max - rect.min;
+
+        render! {
+            <App>
+                <TextureAtlas styles={Some(atlas_styles)} 
+                handle={ui_image_handle} 
+                position={(sign_position.x, sign_position.y)} 
+                tile_size={(sign_size.x, sign_size.y)} 
+                 />
+                <TextureAtlas styles={Some(atlas_styles)} 
+                handle={ui_image_handle} 
+                position={(flower_position.x, flower_position.y)} 
+                tile_size={(flower_size.x, flower_size.y)} 
+                 />
+            </App>
+        }
+    });
+
+    commands.insert_resource(context);
+}
+
+fn main() {
+    BevyApp::new()
+        .insert_resource(WindowDescriptor {
+            width: 1270.0,
+            height: 720.0,
+            title: String::from("UI Example"),
+            ..Default::default()
+        })
+        .add_plugins(DefaultPlugins)
+        .add_plugin(BevyKayakUIPlugin)
+        .add_startup_system(startup)
+        .run();
+}
+