glcpp: Use string_buffer for line continuation removal

glcpp: Use string_buffer for line continuation removal

Migrate removal of line continuations to string_buffer. Before this
it used ralloc_strncat() to append strings, which internally
each time calculates strlen() of its argument. Its argument is
entire shader, so it multiple time scans the whole shader text.

Signed-off-by: Vladislav Egorov <vegorov180@gmail.com>
Tested-by: Dieter Nützel <Dieter at nuetzel-hh.de>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle at amd.com>
V2: Adapt to different API of string buffer (Thomas Helland)