fix: black preview when footage connects directly to a clip

ClipBlock::kBufferIn is declared as NodeValue::kNone with no value
hint, so when a footage node is connected straight to a clip's buffer
input (no effect node in between), the traverser has no type to look
up in the footage's value table and falls back to its last entry. A
footage pushes its video texture first and its audio samples last, so
a video clip ended up fed with audio samples, produced no texture and
the preview went silently black. With any node in between, the table
only carries the passthrough texture, which is why the bug only showed
on direct connections.

Make Node::GetValueHintForInput virtual and override it in ClipBlock
to prefer the value type matching the clip's track (kTexture on video
tracks, kSamples on audio tracks).

Regression tests cover the exact application render path
(PreviewAutoCacher -> RenderManager -> RenderWorkerPool ->
oak-render-worker) for both the direct and indirect cases, the hint
following the track type, and viewer display widgets. The direct-case
test reproduces the black frame without the fix and passes with it.
Full suite: 584 passed.

app/node/node.h

@@ -802,7 +802,8 @@ public:
 		return value_hints_;
 	}
 
-	ValueHint GetValueHintForInput(const QString &input, int element = -1) const
+	virtual ValueHint GetValueHintForInput(const QString &input,
+										   int element = -1) const
 	{
 		return value_hints_.value({ input, element });
 	}