video: hwdec: fallback to hw_ingmtfmt match if now hw_device matches

without this hwdec=rkmpp will fail simply due to the fact that
AV_HWDEVICE_TYPE_RKMPP is a downstream definition and can not be a part
of mpv

video/hwdec.c

@@ -50,6 +50,16 @@ struct mp_hwdec_ctx *hwdec_devices_get_by_imgfmt_and_type(struct mp_hwdec_device
             break;
         }
     }
+    if (!res){
+        // search for an hwdec that can provide the hw_imgfmt as a fallback
+        for (int n = 0; n < devs->num_hwctxs; n++) {
+            struct mp_hwdec_ctx *dev = devs->hwctxs[n];
+            if (dev->hw_imgfmt == hw_imgfmt) {
+                res = dev;
+                break;
+            }
+        }
+    }
     mp_mutex_unlock(&devs->lock);
     return res;
 }