[android] External textures must be rescaled to fill the canvas (#9501)

* [android] External textures must be rescaled to fill the canvas

- After composition, we have a tight box that we need to fill.

- Video decoders typically operate using output buffers that are a whole
  number of blocks (vertically and horizontally). This fixes
  https://github.com/flutter/flutter/issues/34641 where the decoded
  dimensions are rounded up.

- The cropping information for the current frame arrives in the form of
  `SurfaceTexture.getTransformMatrix`. When we apply this transform we
  need to account for the fact that we have already composited. This
  means that in cases where the scaleX, scaleY are less than 1, we need
  to rescale the image. We do this while preserving the aspect ratio.

shell/platform/android/android_external_texture_gl.cc

@@ -68,6 +68,18 @@ void AndroidExternalTextureGL::Paint(SkCanvas& canvas,
   }
 }
 
+// The bounds we set for the canvas are post composition.
+// To fill the canvas we need to ensure that the transformation matrix
+// on the `SurfaceTexture` will be scaled to fill. We rescale and preseve
+// the scaled aspect ratio.
+SkSize ScaleToFill(float scaleX, float scaleY) {
+  const double epsilon = std::numeric_limits<double>::epsilon();
+  // scaleY is negative.
+  const double minScale = fmin(scaleX, fabs(scaleY));
+  const double rescale = 1.0f / (minScale + epsilon);
+  return SkSize::Make(scaleX * rescale, scaleY * rescale);
+}
+
 void AndroidExternalTextureGL::UpdateTransform() {
   JNIEnv* env = fml::jni::AttachCurrentThread();
   fml::jni::ScopedJavaLocalRef<jobject> surfaceTexture =
@@ -77,10 +89,12 @@ void AndroidExternalTextureGL::UpdateTransform() {
   SurfaceTextureGetTransformMatrix(env, surfaceTexture.obj(),
                                    transformMatrix.obj());
   float* m = env->GetFloatArrayElements(transformMatrix.obj(), nullptr);
+  float scaleX = m[0], scaleY = m[5];
+  const SkSize scaled = ScaleToFill(scaleX, scaleY);
   SkScalar matrix3[] = {
-      m[0], m[1], m[2],   //
-      m[4], m[5], m[6],   //
-      m[8], m[9], m[10],  //
+      scaled.fWidth, m[1],           m[2],   //
+      m[4],          scaled.fHeight, m[6],   //
+      m[8],          m[9],           m[10],  //
   };
   env->ReleaseFloatArrayElements(transformMatrix.obj(), m, JNI_ABORT);
   transform.set9(matrix3);