Swap chains, part 1 (#87)

Adds the swap chain interfaces to the API without changing the behavior
of anything else. This includes the C APIs for applications to provide
swap chain implementations. Also adds stub implementations on every
backend.

src/backend/Texture.cpp

@@ -112,6 +112,9 @@ namespace backend {
         if (frozen) {
             return false;
         }
+        if (currentUsage == nxt::TextureUsageBit::Present) {
+            return false;
+        }
         return IsUsagePossible(allowedUsage, usage);
     }