resource: Fix multi-threaded image processing issue

When doing something like this with the same image from a partial used in, say, both the home page and the single page:

```bash
{{ with $img }}
{{ $big := .Fill "1024x512 top" }}
{{ $small := $big.Resize "512x" }}
{{ end }}
```

There would be timing issues making Hugo in some cases try to process the same image with the same instructions in parallel.

You would experience errors of type:

```bash
png: invalid format: not enough pixel data
```

This commit works around that by adding a mutex per image. This should also improve the performance, sligthly, as it avoids duplicate work.

The current workaround before this fix is to always operate on the original:

```bash
{{ with $img }}
{{ $big := .Fill "1024x512 top" }}
{{ $small := .Fill "512x256 top" }}
{{ end }}
```

Fixes #4404

resource/image_cache.go

@@ -28,7 +28,8 @@ type imageCache struct {
 	absCacheDir   string
 	pathSpec      *helpers.PathSpec
 	mu            sync.RWMutex
-	store         map[string]*Image
+
+	store map[string]*Image
 }
 
 func (c *imageCache) isInCache(key string) bool {
@@ -69,6 +70,11 @@ func (c *imageCache) getOrCreate(
 	}
 
 	// Now look in the file cache.
+	// Multiple Go routines can invoke same operation on the same image, so
+	// we need to make sure this is serialized per source image.
+	parent.createMu.Lock()
+	defer parent.createMu.Unlock()
+
 	cacheFilename := filepath.Join(c.absCacheDir, key)
 
 	// The definition of this counter is not that we have processed that amount