Add `ColorImage::from_gray_iter` (#3536)

Add an alternative method for creating a [`ColorImage`] that accepts
`Iterator` as the argument. It can be useful when `&[u8]` is not
available but the iterator is.

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---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/epaint/src/image.rs

@@ -120,6 +120,16 @@ impl ColorImage {
         Self { size, pixels }
     }
 
+    /// Alternative method to `from_gray`.
+    /// Create a [`ColorImage`] from iterator over flat opaque gray data.
+    ///
+    /// Panics if `size[0] * size[1] != gray_iter.len()`.
+    pub fn from_gray_iter(size: [usize; 2], gray_iter: impl Iterator<Item = u8>) -> Self {
+        let pixels: Vec<_> = gray_iter.map(Color32::from_gray).collect();
+        assert_eq!(size[0] * size[1], pixels.len());
+        Self { size, pixels }
+    }
+
     /// A view of the underlying data as `&[u8]`
     #[cfg(feature = "bytemuck")]
     pub fn as_raw(&self) -> &[u8] {