add a depth output

Publish sl::MEASURE::DEPTH as float32 [H, W] millimetres on every frame that
carries image_bgr, with the intrinsics, frame_id and timestamps the other
outputs carry.

The map is copied out rather than published in place: sl::Mat rows can be
padded, so the buffer is not necessarily H * W floats. inf and nan are cleared
to 0 in the same pass, matching the reference driver — a hole is meaningful and
a consumer cannot back-project a non-finite value.

Consumers that need 3D can now back-project depth with the published
intrinsics instead of receiving a point cloud.

Also drops the stale objects entry from the dataflow outputs, missed when
detection was removed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7g2cBJ5e3r15ZbJ9WZn8c

src/zed/camera.h

@@ -38,6 +38,7 @@ public:
     Calibration calibration() const;
     int64_t device_timestamp_ns();
     sl::Mat& image();
+    sl::Mat& depth();
     sl::Mat& point_cloud();
     sl::Camera& handle();
 
@@ -47,6 +48,7 @@ private:
     sl::InitParameters init_params_;
     sl::RuntimeParameters runtime_params_;
     sl::Mat image_mat_;
+    sl::Mat depth_mat_;
     sl::Mat point_cloud_mat_;
 };