ASK MAILING LIST --> glsl: return error when "#pragma debug" receives an invalid argument

GLSL ES 3.00 spec says in chapter 3.4 (Preprocessor), page 12.

   "#pragma allows implementation dependent compiler control. Tokens following
   #pragma are not subject to preprocessor macro expansion. If an
   implementation does not recognize the tokens following #pragma, then it will
   ignore that pragma. [...] The debug pragma
      #pragma debug(on)
      #pragma debug(off)
   can be used to enable compiling and annotating a shader with debug
   information, so that it can be used with a debugger. It can only be used
   outside function definitions. By default, debug is turned off.
   The scope as well as the effect of the optimize and debug pragmas is
   implementation-dependent except that their use must not generate an error."

So it is not clear that we should return an error when there is a pragma like:
   #pragma debug (1.23)

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Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>