glsl: Add constuctors for the common cases of glsl_struct_field

glsl: Add constuctors for the common cases of glsl_struct_field

Fixes a giant pile of GCC warnings:

builtin_types.cpp:60:1: warning: missing initializer for member 'glsl_struct_field::stream' [-Wmissing-field-initializers]

I had to add a default constructor because a non-default constructor
was added.  Otherwise the only constructor would be the one with
parameters, and all the plases like

    glsl_struct_field foo;

would fail to compile.

I wanted to do this in two patches.  All of the initializers of
glsl_struct_field structures had to be converted to use the
constructor because C++ apparently forces you to do one or the other:

builtin_types.cpp:61:1: error: could not convert '{glsl_type::float_type, "near", -1, 0, 0, 0, GLSL_MATRIX_LAYOUT_INHERITED, 0, -1}' from '<brace-enclosed initializer list>' to 'glsl_struct_field'

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>