Adjust PNG encoder color type to match what's read from gl2

This should fix images produced by the -o flag.

src/components/main/compositing/run.rs

@@ -373,7 +373,7 @@ pub fn run_compositor(compositor: &CompositorTask) {
             let img = png::Image {
                 width: width as u32,
                 height: height as u32,
-                color_type: png::RGBA8,
+                color_type: png::RGB8,
                 pixels: pixels,
             };
             let res = png::store_png(&img, &path);