Improve fl_contrast() functionality and documentation

- swap 3rd and 4th parameter of fl_contrast()
- change 'Fl_Fontsize fs' to a neutral 'int size' parameter
- clarify documentation of fl_contrast_level() and that different
  contrast algorithms use different level values
- clarify that level 39 for the CIELAB mode results in "sufficient"
  contrast, i.e. /readable/ text which is the only intention of
  fl_contrast().

Reasoning: swapping the arguments looks more natural, i.e. the context
before another argument (size) is what one would usually expect. Since
the 'size' argument doesn't have be a font size, making it a simple
'int size' is more flexible and correct for future extensions.

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