glsl: Add productions to GLSL grammar for switch statement

glsl: Add productions to GLSL grammar for switch statement

The grammar is modified to support switch statements. Rather than follow the
grammar in the appendix, which allows case labels to be placed ANYWHERE
as a regular statement, we follow the development of the grammar as
described in the body of the GLSL spec.

In this variation, the switch statement has a body which consists of a list
of case statements. A case statement is preceded by a list of case labels and
ends with a list of statements.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>