[GLSL 4.6 Specification] Clarify the translation from UTF-8 scalar values to the corresponding character set tokens

At the GLSL 4.6 specification, add the following paragraph to the 3.1 section:

> The given files for compilation must be in the form of a well-formed UTF-8 code unit sequence. These files are decoded to produce their corresponding sequence of Unicode scalar values. A sequence of character set tokens is then formed by mapping each Unicode scalar value to the corresponding character set token. In the resulting sequence, each pair of characters in the input sequence consisting of U+000D CARRIAGE RETURN followed by U+000A LINE FEED, as well as each U+000D CARRIAGE RETURN not immediately followed by a U+000A LINE FEED, is replaced by a single new-line character.