Move native fwd tx impl into non-duplicated fn
Move native fwd tx impl into non-duplicated fn

Moves the native forward transform code out of the Dim trait
implementation and into a seperate function that does not get
duplicated. No performance difference detected in the changed code.

Compile time and binary size.

Before
2m16.066s
29.5 MiB

After
2m12.034s
28.9 MiB

xiph/rav1e,xiph/rav1e