fix hang on windows in pixel_cast
fix hang on windows in pixel_cast

  - manifested converting double -> int64_t.
  - it is unclear why this explicitness is needed on windows
    to avoid greater calling itself. But it is clear that the
    static_cast<double>(<std::int64_t>) is not enough for the
    template deduction to match the <double,double> case automatically
  - Making this explicit is probably better anyway, but odd it is needed.
  - Finishes closing #2893, refs #2893, amends 96f7120ecc11 and d29a0f18b1fb9c
  - Seen with both VS 2014 CTP 4 and VS 2015 Preview

pramsey/mapnik,tomhughes/mapnik,cjmayo/mapnik,tomhughes/mapnik,lightmare/mapnik,zerebubuth/mapnik,CartoDB/mapnik,mapycz/mapnik,cjmayo/mapnik,lightmare/mapnik,mapnik/mapnik,stefanklug/mapnik,mbrukman/mapnik,Uli1/mapnik,pnorman/mapnik,whuaegeanse/mapnik,naturalatlas/mapnik,mapycz/mapnik,zerebubuth/mapnik,Uli1/mapnik,naturalatlas/mapnik,stefanklug/mapnik,mapycz/mapnik,tomhughes/mapnik,pramsey/mapnik,Uli1/mapnik,cjmayo/mapnik,mbrukman/mapnik,whuaegeanse/mapnik,CartoDB/mapnik,stefanklug/mapnik,cjmayo/mapnik,mapnik/mapnik,pramsey/mapnik,pnorman/mapnik,whuaegeanse/mapnik,CartoDB/mapnik,naturalatlas/mapnik,pnorman/mapnik,mapnik/mapnik,mapnik/mapnik,mbrukman/mapnik,pramsey/mapnik,whuaegeanse/mapnik,Uli1/mapnik,lightmare/mapnik,tomhughes/mapnik,lightmare/mapnik,mbrukman/mapnik,stefanklug/mapnik,naturalatlas/mapnik,zerebubuth/mapnik,pnorman/mapnik