Replace comparisons of FT_Error against 0 with FT_Err_Ok instead.

Most FreeType functions return an FT_Error, which is an int under the hood. But it should always contain one of their enums, not arbitrary values. They define a value for success as FT_Err_Ok, but we have some places where we use its value of 0 instead. This could be confusing as some FreeType functions do return integers (like FT_Get_Char_Index).

So this patch only replaces our comparisons of FT_Error against zero to using FT_Err_Ok instead.