glsl: Add using statements for standard library functions.

glsl: Add using statements for standard library functions.

Standard library functions in C++ are in the std namespace. When using
C++-style header files for the standard library, some compilers, such as
Sun Studio, provide symbols only for the std namespace and not for the
global namespace.

This patch adds using statements for standard library functions. Another
option could have been to prepend standard library function calls with
'std::'.

This patch fixes several compilation errors with Sun Studio.