Change PNG's img.sidecar to ^Info, make img.depth an int.

For compatibility with the upcoming OpenEXR code, img.depth is now an int.
Like OpenEXR's code, it will now also return metadata as ^Info instead of Info.

The example was updated to retrieve the metadata this way.

It regrettably does not fix: #1018. That seems to be a codegen issue in the test runner or elsewhere.

core/image/common.odin

@@ -7,7 +7,7 @@ Image :: struct {
 	width:      int,
 	height:     int,
 	channels:   int,
-	depth:      u8,
+	depth:      int,
 	pixels:     bytes.Buffer,
 	/*
 		Some image loaders/writers can return/take an optional background color.