Allow sparse color attachments

Add implementations and related tests reflecting the spec update.

Bug: dawn:1294
Change-Id: I2c20af313259e1d6d6049189cb8adebe4c2436af
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/81922
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>
diff --git a/dawn.json b/dawn.json
index 41b4fdc..0ee7e22 100644
--- a/dawn.json
+++ b/dawn.json
@@ -1802,7 +1802,7 @@
     "render pass color attachment": {
         "category": "structure",
         "members": [
-            {"name": "view", "type": "texture view"},
+            {"name": "view", "type": "texture view", "optional": true},
             {"name": "resolve target", "type": "texture view", "optional": true},
             {"name": "load op", "type": "load op"},
             {"name": "store op", "type": "store op"},