Effect: glFlush instead of glFinish on tex output

This is much faster (~2-3x) than glFlush. While there's a risk that GL commands
queued to the GL server may not be complete by the time non-GL commands access
the texture, this should be unlikely as we only access the texture from GL.

PiperOrigin-RevId: 527641520

library/effect/src/main/java/com/google/android/exoplayer2/effect/FinalShaderProgramWrapper.java

@@ -360,7 +360,13 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
         outputTexture.fboId, outputTexture.width, outputTexture.height);
     GlUtil.clearOutputFrame();
     checkNotNull(defaultShaderProgram).drawFrame(inputTexture.texId, presentationTimeUs);
-    GLES20.glFinish();
+
+    // glFlush is used here instead of glFinish due to the performance regression that blocking this
+    // thread would do when calling glFinish. As glFlush is non-blocking, it's possible that non-GL
+    // access to the output texture may read stale data (ex. from the prior frame). If we see issues
+    // (ex. flakiness) from glFlush, consider requiring apps reading the texture to call glFinish,
+    // or reconsider using glFinish here.
+    GLES20.glFlush();
     checkNotNull(textureOutputListener).onTextureRendered(outputTexture, presentationTimeUs);
   }