Fixed Image writing out upside down

src/graphics/image.rs

@@ -213,10 +213,18 @@ impl Image {
         let reader = gfx.factory.read_mapping(&dl_buffer)?;
 
         // intermediary buffer to avoid casting.
-        // Apparently this also at one point made the screenshot upside-down,
-        // but no longer?
         let mut data = Vec::with_capacity(self.width as usize * self.height as usize * 4);
-        data.extend(reader.into_iter().flatten());
+        // Assuming OpenGL backend whose typical readback option (glReadPixels) has origin at bottom left.
+        // Image formats on the other hand usually deal with top right.
+        for y in (0..self.height as usize).rev() {
+            data.extend(
+                reader
+                    .iter()
+                    .skip(y * self.width as usize)
+                    .take(self.width as usize)
+                    .flatten(),
+            );
+        }
         Ok(data)
     }
 

src/graphics/mod.rs

@@ -504,9 +504,6 @@ pub fn present(ctx: &mut Context) -> GameResult<()> {
 /// Take a screenshot by outputting the current render surface
 /// (screen or selected canvas) to an `Image`.
 pub fn screenshot(ctx: &mut Context) -> GameResult<Image> {
-    // TODO LATER: This makes the screenshot upside-down form some reason...
-    // Probably because all our images are upside down, for coordinate reasons!
-    // How can we fix it?
     use gfx::memory::Bind;
     let debug_id = DebugId::get(ctx);
 
@@ -1059,5 +1056,4 @@ mod tests {
             assert_relative_eq!(real, expected);
         }
     }
-
 }