sprite: Add custom offsetting mechanism

In case some entity needs its sprite to be offset from its position, add
an `offset` attribute to the `Sprite` component which allows doing just
that.

The offset is a translation vector from the default position.  By
default, a sprite is placed such that its center is located at the
entities `Position`.

src/components/sprite.rs

@@ -2,12 +2,21 @@ use crate::resources;
 
 pub struct Sprite {
     pub image_handle: resources::ImageHandle,
+    pub offset: nalgebra::Vector2<f32>,
 }
 
 impl Sprite {
     pub fn new<S: Into<resources::ImageHandle>>(handle: S) -> Self {
+        Self::with_offset(handle, nalgebra::Vector2::new(0.0, 0.0))
+    }
+
+    pub fn with_offset<S: Into<resources::ImageHandle>>(
+        handle: S,
+        offset: nalgebra::Vector2<f32>,
+    ) -> Self {
         Sprite {
             image_handle: handle.into(),
+            offset,
         }
     }
 }

src/systems/sprite.rs

@@ -10,7 +10,7 @@ pub fn draw_sprites(
     sprite: &components::Sprite,
 ) {
     let spritesize = rendering.get_image_size(&sprite.image_handle).unwrap();
-    let upper_left = position.0 - spritesize / 2.0;
+    let upper_left = position.0 - spritesize / 2.0 + sprite.offset;
     rendering
         .draw_image(
             &sprite.image_handle,