SurfaceTexture: fix a comment

    // mCurrentTextureBuf is the graphic buffer of the current texture. It's

    // possible that this buffer is not associated with any buffer slot, so we

    // must track it separately in order to properly use

    // IGraphicBufferAlloc::freeAllGraphicBuffersExcept.

    // must track it separately in order to support the getCurrentBuffer method.

    sp<GraphicBuffer> mCurrentTextureBuf;

    // mCurrentCrop is the crop rectangle that applies to the current texture.