Replace ecvt() in FCollada with something else

[historic_bruno]

FCollada uses the deprecated POSIX function ​ecvt to convert floats to strings, but it's not available on FreeBSD and it would be nice to have a portable replacement. The result needs to be compatible with the ​COLLADA schema which uses XML's ​xs:double type for floats.

Possible solutions:

• ​std::stringstream, or
• ​snprintf perhaps with the 'g' flag, if it can be massaged into a xs:double-compatible format
• MIT-licensed ecvt replacement: ​http://piumarta.com/software/fcvt/

It's a good idea to avoid ecvt implementations that depend on dtoa, because it's also not available on FreeBSD and there are numerous buggy versions of that code around which apparently ​break with compiler optimizations.