stagefright: don't actually acquire graphic buffer when we don't want to

    ++mNumAvailableUnacquiredBuffers;

    // For BufferQueue we cannot acquire a buffer if we cannot immediately feed it to the codec

    // OR we are discarding this buffer (acquiring and immediately releasing it), which makes

    // UNLESS we are discarding this buffer (acquiring and immediately releasing it), which makes

    // this an ugly logic.

    // NOTE: We could also rely on our debug counter but that is meant only as a debug counter.

    if (!areWeDiscardingAvailableBuffers_l() && mFreeCodecBuffers.empty()) {

        ALOGV("onFrameAvailable: cannot acquire buffer right now, do it later");

        ++mRepeatLastFrameGeneration; // cancel any pending frame repeat

        return;

    }

    VideoBuffer buffer;