glsl/glcpp: Fix glcpp to catch garbage after #if 1 ... #else

glsl/glcpp: Fix glcpp to catch garbage after #if 1 ... #else

Previously, a line such as:

	#else garbage

would flag an error if it followed "#if 0", but not if it followed "#if 1".

We fix this by setting a new bit of state (lexing_else) that allows the lexer
to defer switching to the <SKIP> start state until after the NEWLINE following
the #else directive.

A new test case is added for:

	#if 1
	#else garbage
	#endif

which was untested before, (and did not generate the desired error).

This fixes the following Khronos GLES3 CTS tests:

	tokens_after_else_vertex
        tokens_after_else_fragment

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>