glsl: be more consistent about printing constants

glsl: be more consistent about printing constants

Make sure that we print the same number of digits when printing 0.0 as
any other floating-point number. This will make generating expected
output files for tests easier. To avoid breaking "make check," update
the generated tests for lower_jumps before the next commit which will
bring create_test_cases.py in line with them.

Signed-off-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>