quantize_transform: set whole *pos struct to enable assignment-copy
quantize_transform: set whole *pos struct to enable assignment-copy

get_bitmap_glyph copies this struct via assignment. But to allow
safe assignment-copy, it seems the whole struct must be initialized,
rather than merely each of its members.

Fixes CID 314186 found by Coverity Scan.

astiob/libass,ShiftMediaProject/libass,xbmc/libass,astiob/libass,libass/libass,MrSmile/libass,ShiftMediaProject/libass,libass/libass,xbmc/libass,MrSmile/libass