mediadecoder: allow negative offsets in decoder options

- this is required when screen sharing a secondary screen that is
  aligned to the left or above the primary screen.

Change-Id: I220bf4003ea4bc2071af67a80cb9c5afa0406d5b

src/media/media_device.h

@@ -46,8 +46,8 @@ struct DeviceParams {
     unsigned channel {}; // Channel number
     std::string loop {};
     std::string sdp_flags {};
-    unsigned offset_x {};
-    unsigned offset_y {};
+    int offset_x {};
+    int offset_y {};
     int orientation {};
 };