glsl: Avoid GLboolean vs bool arithmetic MSVC warnings.

glsl: Avoid GLboolean vs bool arithmetic MSVC warnings.

Note that GLboolean is an alias for unsigned char, which lacks the
implicit true/false semantics that C++/C99 bool have.

Reviewed-by: Brian Paul <brianp@vmware.com>
v2: Change gl_shader::IsES and gl_shader_program::IsES to be bool as
recommended by Ian Romanick.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>