Makes egg-based mutagens a bit more achievable

Worked out the math on egg-finding and was rather disconcerted to find
that reptile eggs are so bloody rare.  Assuming you make the veggy roll,
it's 3.8% for reptile eggs.

Lizard isn't *that* good a category, and you shouldn't have to strip a
whole forest of its underbrush to get Raptor or Alpha mutagen.
Scale fixes

Thanks to Ekarus on the IRC for pointing these out.  Not sure when
Lizard got dropped but these should fix it.
Merge pull request #6963 from KA101/MutagenFixes

Makes egg-based mutagens a bit more achievable