perf: insert videoscale before appsink to cut NV12 memmove 6.7×

When hardware decode (mppvideodec/NV12) is active, wrap the appsink in a
GstBin with a videoscale element so the VPU decodes at full stream
resolution but Python only receives a frame pre-scaled to the SDL display
size (default 640x480).

Effect:
  NV12 buffer per frame: 3,133,440 B (1080p) → 460,800 B (640x480)
  memmove per frame:     ~33 ms (80.5% budget) → ~5 ms (expected ~12%)

The videoscale bilinear step runs entirely in software on the A35 cores
but scales down 6.7×, so its cost is far lower than the avoided memmove.
SDL still handles final aspect-ratio fitting inside the viewport, so
visual quality is unchanged relative to what the 640x480 display can show.

Fallback: if videoscale is not available, unscaled NV12 is used as before.

tests/test_player.py

@@ -189,6 +189,30 @@ class FakeMessage:
         return SimpleNamespace(get_name=lambda: self._structure_name)
 
 
+class FakeGhostPad:
+    def __init__(self, name, pad):
+        self.name = name
+    def set_active(self, _):
+        pass
+
+
+class FakeBin:
+    def __init__(self, name=""):
+        self.name = name
+        self._elements = []
+        self._pads = []
+        self.props = {}
+
+    def add(self, elem):
+        self._elements.append(elem)
+
+    def add_pad(self, pad):
+        self._pads.append(pad)
+
+    def set_property(self, name, value):
+        self.props[name] = value
+
+
 class FakeMapFlags:
     READ = 1
 
@@ -204,6 +228,16 @@ class FakeGst:
     SECOND = 1_000_000_000
     MSECOND = 1_000_000
     Caps = SimpleNamespace(from_string=lambda value: value)
+    GhostPad = SimpleNamespace(new=lambda name, pad: FakeGhostPad(name, pad))
+    Bin = SimpleNamespace(new=lambda name="": FakeBin(name))
+
+    @staticmethod
+    def ElementFactory_make(name, alias=None):
+        return None  # signal "not available" so SW fallback kicks in
+
+    # GStreamer uses Gst.ElementFactory.make, exposed as class attribute below
+ElementFactory = SimpleNamespace(make=lambda name, alias=None: None)
+FakeGst.ElementFactory = ElementFactory
 
 
 class _FakeFinfo: