Apply to existing windows (if any)

Our scheme may appears a bit unusual, basically we want the most-common calls AddLine AddRect etc. to not have to perform any check so we always have

Important: does not destruct anything

Important: never called automatically! always explicit.

Works on range [n..n2)

returns results in whatever encoding you request... but note that 2-byte encodings will be BIG-ENDIAN... use stbtt_CompareUTF8toUTF16_bigendian() to c