feat: Add a function for retrieving the window contents

This function is useful for testing the window contents in certain cases. In addition,
this means that we can now have reliable tests for softbuffer's actual functionality.

Signed-off-by: John Nunley <jtnunley01@gmail.com>
Co-authored-by: dAxpeDDa <daxpedda@gmail.com>

src/win32.rs

@@ -202,6 +202,34 @@ impl Win32Impl {
 
         Ok(BufferImpl(self))
     }
+
+    /// Fetch the buffer from the window.
+    pub fn fetch(&mut self) -> Result<Vec<u32>, SoftBufferError> {
+        let buffer = self.buffer.as_ref().unwrap();
+        let temp_buffer = Buffer::new(self.dc, buffer.width, buffer.height);
+
+        // Just go the other way.
+        unsafe {
+            Gdi::BitBlt(
+                temp_buffer.dc,
+                0,
+                0,
+                temp_buffer.width.get(),
+                temp_buffer.height.get(),
+                self.dc,
+                0,
+                0,
+                Gdi::SRCCOPY,
+            );
+        }
+
+        // Flush the operation so that it happens immediately.
+        unsafe {
+            Gdi::GdiFlush();
+        }
+
+        Ok(temp_buffer.pixels().to_vec())
+    }
 }
 
 pub struct BufferImpl<'a>(&'a mut Win32Impl);