OmeroCpp: Miscellaneous fixes

One of the (intentional) changes made when switching to use `cmake` was to enable the same set of strict compiler warnings which are used for bioformats.  There are many problems with OmeroCpp, this PR is just the first of several to correct the most glaring mistakes.  Some are purely cosmetic, others are indicative of design errors, others are real errors.  Combined, they are hiding real issues in the code, so I hope to have eliminated all the more trivial ones soon, making the more serious issues visible.

- Correct a number of compiler warnings (bad syntax, casting, unused parameters, undeclared free functions)
- No functional changes

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Testing:
- jobs should remain green