eframe: Set app icon on Mac and Windows (#2940)

* eframe: Set app icon on Mac and Windows

Also: correctly set window title on Mac when launching from
another process, e.g. python.

Co-authored-by: Wumpf <andreas@rerun.io>

* lint fixes

* Fix web build

* fix typo

* Try fix windows build

---------

Co-authored-by: Wumpf <andreas@rerun.io>

crates/eframe/src/epi/icon_data.rs

@@ -0,0 +1,64 @@
+/// Image data for an application icon.
+///
+/// Use a square image, e.g. 256x256 pixels.
+/// You can use a transparent background.
+#[derive(Clone)]
+pub struct IconData {
+    /// RGBA pixels, with separate/unmultiplied alpha.
+    pub rgba: Vec<u8>,
+
+    /// Image width. This should be a multiple of 4.
+    pub width: u32,
+
+    /// Image height. This should be a multiple of 4.
+    pub height: u32,
+}
+
+impl IconData {
+    /// Convert into [`image::RgbaImage`]
+    ///
+    /// # Errors
+    /// If this is not a valid png.
+    pub fn try_from_png_bytes(png_bytes: &[u8]) -> Result<Self, image::ImageError> {
+        let image = image::load_from_memory(png_bytes)?;
+        Ok(Self::from_image(image))
+    }
+
+    fn from_image(image: image::DynamicImage) -> Self {
+        let image = image.into_rgba8();
+        Self {
+            width: image.width(),
+            height: image.height(),
+            rgba: image.into_raw(),
+        }
+    }
+
+    /// Convert into [`image::RgbaImage`]
+    ///
+    /// # Errors
+    /// If `width*height != 4 * rgba.len()`, or if the image is too big.
+    pub fn to_image(&self) -> Result<image::RgbaImage, String> {
+        let Self {
+            rgba,
+            width,
+            height,
+        } = self.clone();
+        image::RgbaImage::from_raw(width, height, rgba).ok_or_else(|| "Invalid IconData".to_owned())
+    }
+
+    /// Encode as PNG.
+    ///
+    /// # Errors
+    /// The image is invalid, or the PNG encoder failed.
+    pub fn to_png_bytes(&self) -> Result<Vec<u8>, String> {
+        let image = self.to_image()?;
+        let mut png_bytes: Vec<u8> = Vec::new();
+        image
+            .write_to(
+                &mut std::io::Cursor::new(&mut png_bytes),
+                image::ImageOutputFormat::Png,
+            )
+            .map_err(|err| err.to_string())?;
+        Ok(png_bytes)
+    }
+}