Add support for page numbers to VipsScaler.

Needed by I1b9a77a. Note at the moment, it seems like specifying a
page number doesn't work with shrink, so it needs to use im_shrink,
or specify the preconvert option. In my testing, tiff files don't
work very well with vips shrink and non-integer shrink factors,
so only use it for integer shrink factors.

Added unit tests need I4428d38e01 merged into core to work.

Bug: 52045
Change-Id: I2a493ece57601dc613681f67f110a2e7b99c4982
Merge "Add support for page numbers to VipsScaler."