vcl: hide bitmap helpers in an anonymous namespace.

Change-Id: I96ad50f538cfb6ecd256c4268feb81796a135b2b
vcl: create a ScaleContext to share copy/paste Bitmap scaling code.

Change-Id: I5a90b7c6065829d74c61b833f0551b8adc3f2711
vcl: Re-factor to use the ScaleContext to pass parameters.

Change-Id: If4359a6b776b4a94a6934e9fd02ede475a0612a7
vcl: re-factor scaling to use a single fn. pointer based invocation.

Change-Id: Ide61808dbfa82ce67794150f44b6bac4f3c8e302
vcl: parallelize image scaling.

Change-Id: Ia452487c0c8c66a35c4b9fba225348bdef1a27f7