Per-slot Mirror in the camera settings (needs a GL or TextureView path) #226

Split out of #201: a per-slot Mirror (flip left↔right) in the camera settings, next to Turn (PR #225).

Why it is not in #225: the hardware decoder draws straight onto a SurfaceView, and setScaleX(-1) on that view does not mirror the picture on the Aceline AG-216 (Android 10) — the surface is composed by the system, the negative scale is ignored. MediaFormat.KEY_ROTATION has no mirror flag either.

What it needs: a separate drawing path for a mirrored slot, for example

• the decoder renders into a SurfaceTexture and a small GL renderer draws it flipped (the texture transform matrix with a negative X scale), or
• a TextureView for mirrored slots only (ANR on AG-216: RenderThread blocked in eglSwapBuffers with 8 multi-view SurfaceViews #76 moved the single view off TextureView for stability, so it should stay limited to slots that ask for it).

MJPEG on the software path can be mirrored while drawing (a Matrix with preScale(-1, 1)), so it could come first.

Things to keep working: snapshots and long-press screenshots (PixelCopy), the grid, and the frame rate on the TV boxes.