Replace core:image's sidecar with explicit metadata_ptr and metadata_type.

To unpack, use:
```odin

v: ^png.Info;

if img.metadata_ptr != nil && img.metadata_type == png.Info {
	v = (^png.Info)(img.metadata_ptr);
	...
}
```

core/image/png/example.odin

@@ -14,7 +14,7 @@ import "core:os"
 main :: proc() {
 	file: string;
 
-	options := image.Options{};
+	options := image.Options{.return_metadata};
 	err:       compress.Error;
 	img:      ^image.Image;
 
@@ -27,11 +27,11 @@ main :: proc() {
 		fmt.printf("Trying to read PNG file %v returned %v\n", file, err);
 	} else {
 		v: ^png.Info;
-		ok: bool;
 
 		fmt.printf("Image: %vx%vx%v, %v-bit.\n", img.width, img.height, img.channels, img.depth);
 
-		if v, ok = img.sidecar.(^png.Info); ok {
+		if img.metadata_ptr != nil && img.metadata_type == png.Info {
+			v = (^png.Info)(img.metadata_ptr);
 			// Handle ancillary chunks as you wish.
 			// We provide helper functions for a few types.
 			for c in v.chunks {

core/image/png/png.odin

@@ -392,10 +392,7 @@ load_from_stream :: proc(stream: io.Stream, options := Options{}, allocator := c
 		img = new(Image);
 	}
 
-	info: ^Info;
-	if img.sidecar == nil {
-		info = new(Info);
-	}
+	info := new(Info, context.allocator);
 
 	ctx := &compress.Context{
 		input = stream,
@@ -417,7 +414,6 @@ load_from_stream :: proc(stream: io.Stream, options := Options{}, allocator := c
 
 	header:	IHDR;
 
-	img.sidecar = info;
 	info.chunks.allocator = context.temp_allocator;
 
 	// State to ensure correct chunk ordering.
@@ -659,7 +655,8 @@ load_from_stream :: proc(stream: io.Stream, options := Options{}, allocator := c
 	}
 
 	if .return_header in options || .return_metadata in options {
-		img.sidecar = info;
+		img.metadata_ptr  = info;
+		img.metadata_type = typeid_of(Info);
 	}
 	if .do_not_decompress_image in options {
 		img.channels = final_image_channels;