Add front camera texture coordinate handling

- Add separate texture coordinates for front and back cameras
- Front camera uses mirrored coordinates for natural selfie view
- Add setFrontCamera() method to renderer for dynamic switching
- Update texture coord buffer on GL thread when camera changes
- CameraScreen observes isFrontCamera state to trigger updates

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/src/main/java/no/naiv/tiltshift/ui/CameraScreen.kt

@@ -125,6 +125,12 @@ fun CameraScreen(
         glSurfaceView?.requestRender()
     }
 
+    // Update renderer when camera switches (front/back)
+    LaunchedEffect(isFrontCamera) {
+        renderer?.setFrontCamera(isFrontCamera)
+        glSurfaceView?.requestRender()
+    }
+
     // Start camera when surface texture is available
     LaunchedEffect(surfaceTexture) {
         surfaceTexture?.let {