Fix integer division in transform.cpp

[springmeyer]

Ran mbgl-native through coverity static analysis tonight. Coverity uncovered that our transform code has been doing integer division. This may be leading to loss of precision / truncated values - a classic C/C++ bug. If anyone is not familiar with this behavior have a look at: http://coliru.stacked-crooked.com/a/62dd1db484efeac8

The report looked like:

Fuller report at https://scan4.coverity.com/reports.htm#v13662/p11099 (you should be able to login via github account).

Overall: I hit two coverity bugs that prevented most files from being able to be processed that I've emailed coverity support about. It looks like coverity does not support atomic and there are some libc++ coverity issues. I'll post more bug reports when I can process the entire libmgbl-core.

[incanus]

Nice @springmeyer, this is enlightening.

[springmeyer]

btw, you would think that -Wsign-conversion or -Wconversion would warn on this, but it does not in all cases (http://stackoverflow.com/questions/6076878/locating-numerical-errors-due-to-integer-division). So, we need to be smart about always using explicitly using static_cast when changing types. This will help prevent problems at the time you write the code and especially when others copy and paste to refactor.