tests: major coverage round for node/render/audio/plugin subsystems

Add 12 gtest files (~400 tests) covering previously untested or
under-tested areas:
- node_math_test: MathNode operations across number/rational/vector/
  matrix/color/sample pairings, shader code generation
- node_undo_test: all nodeundo command classes redo/undo
- track_test: Track block management, lookup, references, Value()
- render_diskcache_test: FrameHashCache EXR/JPEG round trips,
  DiskManager LRU eviction, state persistence
- node_group_test: NodeGroup passthrough registration and serialization
- plugin_paraminstance_test: OFX param instances and clip image logic
- audio_waveform_test: AudioVisualWaveform + AudioProcessor
- node_value_extended_test: NodeValue conversions, NodeValueTable ops,
  NodeKeyframe/bezier behavior
- render_projectcopier_test: ProjectCopier sync, PlaybackCache,
  AudioPlaybackCache PCM segments
- node_core_test: Node input arrays, flags, contexts, links, keyframe
  events, CopyInputs
- clip_traverser_test: ClipBlock speed/reverse/loop time mapping,
  traverser time propagation
- audio_manager_viewer_test: AudioManager device API, ViewerOutput
  params/streams/signals

Also fixes two real bugs found by the new tests:
- MathNode vec-vec divide crashed (debug) or produced NaN (release) on
  the zero padding components of vec2/vec3 operands
- NodeKeyframe's default constructor left previous_/next_ and the
  bezier handles uninitialized

app/node/keyframe.cpp

@@ -46,8 +46,14 @@ NodeKeyframe::NodeKeyframe(const rational &time, const QVariant &value,
 }
 
 NodeKeyframe::NodeKeyframe()
+	: type_(NodeKeyframe::kLinear)
+	, bezier_control_in_(QPointF(0.0, 0.0))
+	, bezier_control_out_(QPointF(0.0, 0.0))
+	, track_(-1)
+	, element_(-1)
+	, previous_(nullptr)
+	, next_(nullptr)
 {
-	type_ = NodeKeyframe::kLinear;
 }
 
 NodeKeyframe::~NodeKeyframe()