Consistently use the compat wrappers for file IO

As long as WelsFileHandle* is equal to FILE* this doesn't matter,
but for consistency use the WelsF* functions for all handles
opened by WelsFopen, and use WelsFileHandle* as type for it
instead of FILE*.
Merge pull request #243 from mstorsjo/use-stdio-wrappers

Consistently use the compat wrappers for file IO