CameraX's Preview use case builds the SurfaceTexture transform
matrix with a horizontal mirror baked in for the front camera (so
the selfie preview reads naturally without consumer effort). Flutter
applies that matrix when sampling the texture. Adding our own
`Transform.flip(flipX: true)` on top double-mirrors — which on its
own would just un-mirror the selfie, but combined with CameraX's
counter-rotation when the device tilts, it makes the rotation appear
to *follow* the phone (i.e. tilt CW 90° → preview goes 90° more
CW). Removing the Flutter flip lets CameraX's matrix do the work on
its own.
iOS keeps the Flutter flip: AVCaptureConnection.isVideoMirrored on
the data output is `false` there (capture-time mirror would land
in the recorded MP4), so the preview-only mirror must live in the
widget tree.
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