PERF: Avoid allocation induced by VLV pixels

The `DifferenceImageFilter` is often used on `VectorImage`s, in particular when
doing `compare-image`.

Creating new `VariableLengthVector` pixels at each iteration, or casting them
will automatically end up in construction and destruction of VLV pixels, and
thus allocation and liberation of heap memory. This is not efficient.

This commit aims at minimizing the number of allocation by factorizing out
everything that can be (pixel size, typical max value, typical zero value).
Resetting value is done by assignment, which has the good property of not
inducing any allocation on VLV variables.