Overhaul handling of NaN/Inf values in ind2X functions (bug #56950)

Exceptional values (NaN, Inf) are now mapped to the last value in the colormap rather than throwing an error.

NEWS.11.md: Announce change in Matlab Compatibility section.
ind2x.m: Split input validation of whether X is an indexed image. New variable “do_warn” to track whether any out-of-bounds indices have been found anywhere in function. Replace NaN values with Inf in input X to force mapping to last value in colormap. Check “do_warn” at end of function and issue warning about out-of-bounds indices just once.
ind2rgb.m: Add Programming Note to docstring explaining how values are clipped to colormap. Change error() message for unreachable code to ask users to report error to bugs.octave.org. Add bug numbers to specific BIST tests. Add BIST test for bug #56950. Remove all but one test which implicitly tests iscolormap() [there are already BIST tests in that m-file].
ind2gray.m: Add Programming Note to docstring explaining how values are clipped to colormap. Remove all but one BIST test which implicitly tests iscolormap(). Add BIST test with input NaN value.
iscolormap.m: Remove test for isnumeric() as condition of being a colormap. Replace test “ndims == 2” with ismatrix(). Add test “! isempty”. Add BIST test for empty colormap input.
im2frame.m: Reflow DOCSTRING to 80-characters. Correct error message and first input. Add FIXME note about missing input validation of input class. Add BIST tests for input validation.

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