Change metal format to BGRA8Unorm

This should fix channel swizzling on M1 and restore functionality on Intel
diff --git a/piet-gpu-hal/src/metal.rs b/piet-gpu-hal/src/metal.rs
index 307def8..03a23b0 100644
--- a/piet-gpu-hal/src/metal.rs
+++ b/piet-gpu-hal/src/metal.rs
@@ -350,7 +350,7 @@
         //desc.set_mipmap_level_count(1);
         let mtl_format = match format {
             ImageFormat::A8 => metal::MTLPixelFormat::R8Unorm,
-            ImageFormat::Rgba8 => metal::MTLPixelFormat::RGBA8Unorm,
+            ImageFormat::Rgba8 => metal::MTLPixelFormat::BGRA8Unorm,
         };
         desc.set_pixel_format(mtl_format);
         desc.set_usage(metal::MTLTextureUsage::ShaderRead | metal::MTLTextureUsage::ShaderWrite);