gpu: add and track fields needed by the compute renderer

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/caches.go

@@ -26,8 +26,10 @@ type opCache struct {
 }
 
 type opCacheValue struct {
-	data   pathData
-	bounds f32.Rectangle
+	data pathData
+	// cpuData is the retained data, if kept by the compute renderer.
+	cpuData []byte
+	bounds  f32.Rectangle
 	// the fields below are handled by opCache
 	key  ops.Key
 	keep bool