avcodec/movtextenc: fix writing to bytestream on BE arches

avcodec/movtextenc: fix writing to bytestream on BE arches

Fixes fate-binsub-movtextenc on PPC64

Currently tags are written in reverse order on BE arches. This is fixed
by using MKBETAG() and AV_RB32() to be arch agnostics.

Also s->font_count is of type int. On BE arches with 32bit int,
count = AV_RB16(&s->font_count) will read two most significant bytes
instead of the least significant bytes. This is fixed by assigning
s->font_count to count first.

The final change is modifying the type of len. On BE arches
the most significant byte of the int was written instead of the least
significant byte.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>