Added offset parameter to TextureAtlas::from_grid_with_padding (bevyengine#4836)

# Objective
Increase compatibility with a fairly common format of padded spritesheets, in which half the padding value occurs before the first sprite box begins. The original behaviour falls out when `Vec2::ZERO` is used for `offset`.

See below unity screenshot for an example of a spritesheet with padding

![Screen Shot 2022-05-24 at 4 11 49 PM](https://user-images.githubusercontent.com/30442265/170123682-287e5733-b69d-452b-b2e6-46d8d29293fb.png)

## Solution
Tiny change to `crates/bevy_sprite/src/texture_atlas.rs`

## Migration Guide

Calls to `TextureAtlas::from_grid_with_padding` should be modified to include a new parameter, which can be set to `Vec2::ZERO` to retain old behaviour.
```rust
from_grid_with_padding(texture, tile_size, columns, rows, padding)
                                  |
                                  V
from_grid_with_padding(texture, tile_size, columns, rows, padding, Vec2::ZERO)
```


Co-authored-by: FraserLee <30442265+FraserLee@users.noreply.github.com>

Author: FraserLee

crates/bevy_sprite/src/texture_atlas.rs

@@ -68,25 +68,27 @@ impl TextureAtlas {
     }
 
     /// Generate a `TextureAtlas` by splitting a texture into a grid where each
-    /// cell of the grid  of `tile_size` is one of the textures in the atlas
+    /// `tile_size` by `tile_size` grid-cell is one of the textures in the atlas
     pub fn from_grid(
         texture: Handle<Image>,
         tile_size: Vec2,
         columns: usize,
         rows: usize,
     ) -> TextureAtlas {
-        Self::from_grid_with_padding(texture, tile_size, columns, rows, Vec2::new(0f32, 0f32))
+        Self::from_grid_with_padding(texture, tile_size, columns, rows, Vec2::ZERO, Vec2::ZERO)
     }
 
     /// Generate a `TextureAtlas` by splitting a texture into a grid where each
-    /// cell of the grid of `tile_size` is one of the textures in the atlas and is separated by
-    /// some `padding` in the texture
+    /// `tile_size` by `tile_size` grid-cell is one of the textures in the
+    /// atlas. Grid cells are separated by some `padding`, and the grid starts
+    /// at `offset` pixels from the top left corner.
     pub fn from_grid_with_padding(
         texture: Handle<Image>,
         tile_size: Vec2,
         columns: usize,
         rows: usize,
         padding: Vec2,
+        offset: Vec2,
     ) -> TextureAtlas {
         let mut sprites = Vec::new();
         let mut x_padding = 0.0;
@@ -102,8 +104,8 @@ impl TextureAtlas {
                 }
 
                 let rect_min = Vec2::new(
-                    (tile_size.x + x_padding) * x as f32,
-                    (tile_size.y + y_padding) * y as f32,
+                    (tile_size.x + x_padding) * x as f32 + offset.x,
+                    (tile_size.y + y_padding) * y as f32 + offset.y,
                 );
 
                 sprites.push(Rect {