LibGfx/PNG: Collect tRNS (transparency chunk) info for all color types

The tRNS metadata is valid for color types 0, 2 and 3, so let's collect
it in each of those cases.

This ensures that we produce a bitmap with an alpha channel if needed.

Userland/Libraries/LibGfx/PNGLoader.cpp

@@ -834,6 +834,8 @@ static bool process_PLTE(ReadonlyBytes data, PNGLoadingContext& context)
 static bool process_tRNS(ReadonlyBytes data, PNGLoadingContext& context)
 {
     switch (context.color_type) {
+    case 0:
+    case 2:
     case 3:
         context.palette_transparency_data.append(data.data(), data.size());
         break;