rename inner selector in seletor to subselector

[toge]

Thank you for relaxing compiler requirements on lunasvg/2.3.9.

I met compilation error on gcc 5.4.0.

source/parser.cpp: In static member function ‘static bool lunasvg::RuleData::matchPseudoClassSelector(const lunasvg::PseudoClassSelector&, const lunasvg::Element*)’:
source/parser.cpp:1229:30: error: use of ‘selector’ before deduction of ‘auto’
         for(auto& selector : selector.subSelectors) {
                              ^
source/parser.cpp:1230:29: error: unable to deduce ‘auto&&’ from ‘selector’
             for(auto& sel : selector) {
                             ^
source/parser.cpp:1241:30: error: use of ‘selector’ before deduction of ‘auto’
         for(auto& selector : selector.subSelectors) {
                              ^
source/parser.cpp:1242:29: error: unable to deduce ‘auto&&’ from ‘selector’
             for(auto& sel : selector) {
                             ^

Those errors are caused by same symbol name in for loops.
If possible, woudl you merge this PR to work well on old gcc compilers.

[sammycage]

Thank you 🎉🎉🎉