cppcheck: some cleanings

[libavogadro/src/extensions/cartesianextension.cpp:406]: (performance) Prefer prefix ++/-- operators for non-primitive types.
[libavogadro/src/extensions/surfaces/openqube/gaussianset.cpp:354] -> [libavogadro/src/extensions/surfaces/openqube/gaussianset.cpp:356]: (warning) Variable 'skip' is reassigned a value before the old one has been used. 'break;' missing?
(+ other similar warnings)
[libavogadro/src/python/sip.cpp:406]: (style) Statements following return, break, continue, goto or throw will never be executed.
[libavogadro/src/textrenderer_p.cpp:215]: (error) Memory leak: rawbitmap
[libavogadro/src/textrenderer_p.cpp:249]: (error) Memory leak: glyphbitmap
(+ other related)