Mention alpha for matrix transformation background color.

PiperOrigin-RevId: 453633920
(cherry picked from commit d5e4faa9)

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/GlMatrixTransformation.java

@@ -27,7 +27,8 @@ import java.io.IOException;
  * axes). Transformed pixels that are moved outside of the normal device coordinate range are
  * clipped.
  *
- * <p>Output frame pixels outside of the transformed input frame will be black.
+ * <p>Output frame pixels outside of the transformed input frame will be black, with alpha = 0 if
+ * applicable.
  */
 public interface GlMatrixTransformation extends GlEffect {
   /**

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/MatrixTransformation.java

@@ -24,7 +24,8 @@ import android.graphics.Matrix;
  * axes). Transformed pixels that are moved outside of the normal device coordinate range are
  * clipped.
  *
- * <p>Output frame pixels outside of the transformed input frame will be black.
+ * <p>Output frame pixels outside of the transformed input frame will be black, with alpha = 0 if
+ * applicable.
  */
 public interface MatrixTransformation extends GlMatrixTransformation {
   /**

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/MatrixTransformationProcessor.java

@@ -36,7 +36,7 @@ import java.util.Arrays;
  * Transformed vertices that are moved outside of this range after any of the transformation
  * matrices are clipped to the NDC range.
  *
- * <p>The background color of the output frame will be black.
+ * <p>The background color of the output frame will be (r=0, g=0, b=0, a=0).
  */
 @SuppressWarnings("FunctionalInterfaceClash") // b/228192298
 /* package */ final class MatrixTransformationProcessor extends SingleFrameGlTextureProcessor {

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/Presentation.java

@@ -39,7 +39,7 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
  *
  * <p>Cropping or aspect ratio is applied before setting resolution.
  *
- * <p>The background color of the output frame will be black.
+ * <p>The background color of the output frame will be black, with alpha = 0 if applicable.
  */
 public final class Presentation implements MatrixTransformation {
 

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/ScaleToFitTransformation.java

@@ -31,7 +31,7 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
  * <p>All input frames' pixels will be preserved and copied into an output frame, potentially
  * changing the width and height of the frame by scaling dimensions to fit.
  *
- * <p>The background color of the output frame will be black.
+ * <p>The background color of the output frame will be black, with alpha = 0 if applicable.
  */
 public final class ScaleToFitTransformation implements MatrixTransformation {