glsl: respect the source number set by #line <line> <source>

glsl: respect the source number set by #line <line> <source>

From GLSL 1.30.10, section 3.3 (Preprocessor):
"#line line source-string-number ... After processing this directive
(including its new-line), the implementation will behave as if it is
compiling at ... source string number source-string-number. Subsequent
source strings will be numbered sequentially, until another #line
directive overrides that numbering."

In the previous implementation the source number was always zero.
Subsequent source strings are still not numbered sequentially, because
in the glShaderSource implementation we are concatenating the source code
strings into one long string.

Partially fixes https://bugs.freedesktop.org/show_bug.cgi?id=88815

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