<enhanced:img retina> support

Describe the problem

@sveltejs/enhanced-img renders the img tag using intrinsic dimensions, which causes incorrect rendering if the image is a 2x retina image.

Describe the proposed solution

I propose a boolean prop, <enhanced:img retina> that will set the img width/height to the correct logical dimensions: 1/2 the intrinsic dimensions.

Alternatives considered

• CSS styling is not helpful because it puts the burden back on the developer to specify sizes explicitly

Importance

nice to have

Additional Information

Here is a crude patch just to show the essence:

diff --git a/src/preprocessor.js b/src/preprocessor.js
index 0222037e593b554cae5b8712fa8c8e51fd071932..051d57097d4844ee3474b2f0d60584ca6578f8ba 100644
--- a/src/preprocessor.js
+++ b/src/preprocessor.js
@@ -304,8 +306,8 @@ function img_to_picture(consts, content, node, image) {
 
 	res += `<img ${serialize_img_attributes(content, attributes, {
 		src: to_value(consts, image.img.src),
-		width: image.img.w,
-		height: image.img.h
+		width: image.img.w/2,
+		height: image.img.h/2
 	})} />`;
 
 	return (res += '</picture>');
@@ -345,8 +347,8 @@ function dynamic_img_to_picture(content, node, src_var_name) {
 
 	const details = {
 		src: `{${src_var_name}.img.src}`,
-		width: `{${src_var_name}.img.w}`,
-		height: `{${src_var_name}.img.h}`
+		width: `{${src_var_name}.img.w/2}`,
+		height: `{${src_var_name}.img.h/2}`
 	};
 
 	return `{#if typeof ${src_var_name} === 'string'}

Start in src/preprocessor.js, focusing on img_to_picture and dynamic_img_to_picture, where the issue's proposed changes adjust generated image dimensions. Trace how the enhanced:img retina prop would be recognized, then verify that both static and dynamic images receive logical dimensions while other images remain unchanged.