glsl/gs: add gl_in support to builtin_variables.cpp.

glsl/gs: add gl_in support to builtin_variables.cpp.

Previously, builtin_variables.cpp was written assuming that we
supported ARB_geometry_shader4 style geometry shader inputs, meaning
that each built-in varying input to a geometry was supplied via an
array variable whose name ended in "In", e.g. gl_PositionIn or
gl_PointSizeIn.

However, in GLSL 1.50 style geometry shaders, things work
differently--built-in inputs are supplied to geometry shaders via a
built-in interface block called gl_in, which contains all the built-in
inputs using their usual names (e.g. the gl_Position input is supplied
to the geometry shader as gl_in[i].gl_Position).

This patch adds the necessary logic to builtin_variables.cpp to create
the gl_in interface block and populate it accordingly for geometry
shader inputs.  The old ARB_geometry_shader4 style varyings are
removed, though they can easily be added back in the future if we
decide to support ARB_geometry_shader4.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>