glsl/glcpp: Fix to emit spaces following directives

glsl/glcpp: Fix to emit spaces following directives

The glcpp lexer and parser use the space_tokens state bit to avoid emitting
tokens for spaces while parsing a directive. Previously, this bit was only
being set again by the first non-space token following a directive.

This led to a bug where a space, (or a comment that should emit a space),
immediately following a directive, (optionally searated by newlines), would be
omitted from the output.

Here we fix the bug by also setting the space_tokens bit whenever we lex a
newline in the standard start conditions.