Remove some unused macros
Move the definition of WELS_ALIGN out from the #if

Both versions of WELS_ALIGN are identical, so there's no need to
be able to switch between the two.
Remove an unnecessary define of inline for APPLE_IOS

The standard inline keyword is supported just fine when building
for iOS, there's no need to define it there.
Bundle all MSVC/GCC alignment macros together

This reduces the number of lines spent on having the same
ifdef conditions over and over.
Clarify ifdefs around implementations of ENDIAN_FIX
Remove a duplicate median function

The decoder used WelsMedian while the encoder used WELS_MEDIAN.
The former has two different implementations, WELS_MEDIAN was
identical to the disabled version of WelsMedian.

Settle on using the same implementation for both decoder and
encoder - whichever version of the implementations is faster
should be used for both.